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