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.

system/kernel/architecture/x86_64/paging.zig

@@ -10,10 +10,10 @@
 //! Everything is 4 KiB pages — precise and simple; the extra table memory is
 //! negligible against available RAM.
 
-const danos = @import("danos");
+const system = @import("system");
 const io = @import("io.zig");
 
-const page_size = danos.page_size;
+const page_size = system.page_size;
 
 // Page-table entry bits.
 const present: u64 = 1 << 0;
@@ -58,7 +58,7 @@ const bootstrap_physmap_limit: u64 = 4 << 30;
 /// both the loader's bootstrap tables and the kernel's own, which share the
 /// physmap base.
 fn tableAt(physical: u64) *[512]u64 {
-    return @ptrFromInt(danos.physicalToVirtual(physical));
+    return @ptrFromInt(system.physicalToVirtual(physical));
 }
 
 fn allocTable() u64 {
@@ -102,12 +102,12 @@ fn mapRangePhysmap(pml4: u64, physical_base: u64, len: u64, flags: u64) void {
     var address = physical_base & ~@as(u64, page_size - 1);
     const end = physical_base + len;
     while (address < end) : (address += page_size) {
-        mapPage(pml4, danos.physicalToVirtual(address), address, flags);
+        mapPage(pml4, system.physicalToVirtual(address), address, flags);
     }
 }
 
-fn regions(mm: danos.MemoryMap) []const danos.MemoryRegion {
-    return @as([*]const danos.MemoryRegion, @ptrFromInt(danos.physicalToVirtual(mm.regions)))[0..mm.len];
+fn regions(mm: system.MemoryMap) []const system.MemoryRegion {
+    return @as([*]const system.MemoryRegion, @ptrFromInt(system.physicalToVirtual(mm.regions)))[0..mm.len];
 }
 
 /// Enable the NX bit in the page-table format (EFER.NXE). Must happen before we
@@ -118,7 +118,7 @@ fn enableNx() void {
 }
 
 /// Build the address space and switch onto it.
-pub fn init(allocFrame: *const fn () ?u64, freeFrame: *const fn (u64) void, boot_information: *const danos.BootInformation) void {
+pub fn init(allocFrame: *const fn () ?u64, freeFrame: *const fn (u64) void, boot_information: *const system.BootInformation) void {
     alloc_frame = allocFrame;
     free_frame = freeFrame;
     enableNx();
@@ -136,7 +136,7 @@ pub fn init(allocFrame: *const fn () ?u64, freeFrame: *const fn (u64) void, boot
     //    the kernel touches directly), RW + NX.
     const fb = boot_information.framebuffer;
     mapRangePhysmap(pml4, fb.base, @as(u64, fb.height) * fb.pitch, present | writable | no_execute);
-    mapPage(pml4, danos.physicalToVirtual(0xFEE00000), 0xFEE00000, present | writable | no_execute);
+    mapPage(pml4, system.physicalToVirtual(0xFEE00000), 0xFEE00000, present | writable | no_execute);
 
     // 3. The kernel's own segments at their higher-half link addresses, mapped
     //    to their low physical load addresses with real ELF permissions: code
@@ -206,11 +206,11 @@ pub fn mapMmio(physical: u64, len: u64, writable_page: bool) u64 {
     const last = physical + (if (len == 0) 1 else len) - 1;
     var address = first;
     while (address <= (last & ~@as(u64, page_size - 1))) : (address += page_size) {
-        const virtual = danos.physicalToVirtual(address);
+        const virtual = system.physicalToVirtual(address);
         mapPage(kernel_pml4, virtual, address, flags);
         invalidate(virtual);
     }
-    return danos.physicalToVirtual(physical);
+    return system.physicalToVirtual(physical);
 }
 
 /// Like `descend`, but also sets the U/S bit on the intermediate entry (new or