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/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;