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/heap.zig

@@ -13,11 +13,11 @@
 //! interrupt handlers (ours don't). A lock comes with threads/SMP.
 
 const std = @import("std");
-const danos = @import("danos");
+const system = @import("system");
 const architecture = @import("architecture");
 const pmm = @import("pmm.zig");
 
-const page_size = danos.page_size;
+const page_size = system.page_size;
 
 /// Virtual base of the heap: the start of the higher half, which is unmapped and
 /// well clear of the identity-mapped low half. (Canonical on x86_64; an architecture that