Rename the shared contract module danos -> system; QEMU logs to /var/log/system

The shared kernel<->user ABI contract (BootInformation, the SystemCall numbers,
DeviceDescriptor, page_size, ...) is now the `system` module at
system/system.zig, following the convention that a directory's root file takes
the directory's name.

One overlap to note: the runtime's syscall wrappers are already `runtime.system`,
so the single file that uses both the contract and those wrappers
(library/runtime/heap.zig) aliases the wrappers locally as `system_calls`. The
two are distinct (top-level `system` vs `runtime.system`); everywhere else the
contract is just `system`.

Also: the QEMU run's serial capture now lands in the FHS log location,
zig-out/var/log/system/serial0-<timestamp>.log — a stand-in for the kernel's own
logging system, which will eventually write there itself.

Suite 35/35 plus host tests green.

library/runtime/device.zig

@@ -3,13 +3,13 @@
 //! ownership of its hardware; the claim is the capability the kernel checks before
 //! mapping registers or routing an IRQ.
 
-const danos = @import("danos");
+const system = @import("system");
 const sc = @import("system-call.zig");
 
-pub const DeviceDescriptor = danos.DeviceDescriptor;
-pub const ResourceDescriptor = danos.ResourceDescriptor;
-pub const DeviceClass = danos.DeviceClass;
-pub const ResourceKind = danos.ResourceKind;
+pub const DeviceDescriptor = system.DeviceDescriptor;
+pub const ResourceDescriptor = system.ResourceDescriptor;
+pub const DeviceClass = system.DeviceClass;
+pub const ResourceKind = system.ResourceKind;
 
 inline fn failed(r: usize) bool {
     return r > ~@as(usize, 0) - 4095;
@@ -33,7 +33,7 @@ pub fn mmioMap(device_id: u64, resource_index: u64) ?usize {
 }
 
 /// `DeviceDescriptor.parent` for a device with no parent.
-pub const no_parent = danos.no_parent;
+pub const no_parent = system.no_parent;
 
 /// Publish `descriptor` as a child of `parent_id`, which this process must have claimed.
 /// Returns the new device id. The child is left unclaimed, so whichever driver owns

library/runtime/heap.zig

@@ -13,10 +13,10 @@
 //! lock and larger alignments come when user programs gain threads.
 
 const std = @import("std");
-const danos = @import("danos");
-const system = @import("system.zig");
+const system = @import("system");
+const system_calls = @import("system.zig");
 
-const page_size = danos.page_size;
+const page_size = system.page_size;
 
 /// A block header, at the start of every block; while free it also links the
 /// free list via `next`.
@@ -46,8 +46,8 @@ fn payloadOf(block: *Block) [*]u8 {
 /// grants usually are adjacent). Returns false if the kernel is out of memory.
 fn grow(minimum_bytes: usize) bool {
     const bytes = alignUp(@max(minimum_bytes, chunk), page_size);
-    const ret = system.mmap(bytes, system.PROT_READ | system.PROT_WRITE);
-    if (system.mmapFailed(ret)) return false;
+    const ret = system_calls.mmap(bytes, system_calls.PROT_READ | system_calls.PROT_WRITE);
+    if (system_calls.mmapFailed(ret)) return false;
 
     const block: *Block = @ptrFromInt(ret);
     block.size = bytes;

library/runtime/ipc.zig

@@ -3,7 +3,7 @@
 //! reached this way. The server side (`replyWait`, which returns two values) is
 //! added with the first server binary.
 
-const danos = @import("danos");
+const system = @import("system");
 const sc = @import("system-call.zig");
 
 /// A small-int handle into the calling process's handle table.
@@ -30,13 +30,13 @@ pub fn createEndpoint() ?Handle {
 }
 
 /// Publish endpoint `h` under a well-known service id so other processes find it.
-pub fn register(id: danos.ServiceId, h: Handle) bool {
+pub fn register(id: system.ServiceId, h: Handle) bool {
     return !failed(sc.systemCall2(.ipc_register, @intFromEnum(id), h));
 }
 
 /// Find the endpoint published under `id`, installing a handle to it in this
 /// process.
-pub fn lookup(id: danos.ServiceId) ?Handle {
+pub fn lookup(id: system.ServiceId) ?Handle {
     const r = sc.systemCall1(.ipc_lookup, @intFromEnum(id));
     return if (failed(r)) null else r;
 }
@@ -53,7 +53,7 @@ pub fn call(h: Handle, message: []const u8, reply: []u8) CallError!usize {
 /// Set in `Received.badge` when what arrived is an asynchronous notification — a
 /// bound device interrupt — rather than a client's message. The low bits carry the
 /// GSI. See `isNotification`.
-pub const notify_badge_bit: u64 = danos.notify_badge_bit;
+pub const notify_badge_bit: u64 = system.notify_badge_bit;
 
 /// The result of a `replyWait`: the request length and the sender's badge (a
 /// task id, or an IRQ notification if the high bit is set).
@@ -84,7 +84,7 @@ pub fn replyWait(h: Handle, reply: []const u8, receive: []u8) Received {
     asm volatile ("syscall"
         : [rax] "={rax}" (rax),
           [rdx] "+{rdx}" (rdx),
-        : [n] "{rax}" (@intFromEnum(danos.SystemCall.ipc_reply_wait)),
+        : [n] "{rax}" (@intFromEnum(system.SystemCall.ipc_reply_wait)),
           [a0] "{rdi}" (h),
           [a1] "{rsi}" (@intFromPtr(reply.ptr)),
           [a3] "{r10}" (@intFromPtr(receive.ptr)),

library/runtime/system-call.zig

@@ -6,8 +6,8 @@
 //! Note argument #3 goes in **r10, not rcx** — rcx is unavailable across the
 //! instruction, so the kernel reads the 4th argument from r10.
 
-const danos = @import("danos");
-const SystemCall = danos.SystemCall;
+const system = @import("system");
+const SystemCall = system.SystemCall;
 
 pub inline fn systemCall0(n: SystemCall) usize {
     return asm volatile ("syscall"

library/runtime/system.zig

@@ -1,15 +1,15 @@
 //! Typed system_call surface for user space — thin wrappers over the raw `system_call`
-//! stubs, one per kernel call. Numbers come from `danos.SystemCall`, the single
+//! stubs, one per kernel call. Numbers come from `system.SystemCall`, the single
 //! source of truth shared with the kernel dispatcher.
 
-const danos = @import("danos");
+const system = @import("system");
 const sc = @import("system-call.zig");
 
 /// `mmap` protection flags (matching the usual C bit values). Grants are always
 /// readable+writable today; the kernel does not yet honour finer prot.
-pub const PROT_READ: usize = danos.prot_read;
-pub const PROT_WRITE: usize = danos.prot_write;
-pub const PROT_EXEC: usize = danos.prot_exec;
+pub const PROT_READ: usize = system.prot_read;
+pub const PROT_WRITE: usize = system.prot_write;
+pub const PROT_EXEC: usize = system.prot_exec;
 
 /// Give up the rest of this quantum.
 pub fn yield() void {