Initial commit: Schunix as it stands before the SchOS rename

An x86-64 operating system built from xv6, now largely rewritten: SMP,
a copy-on-write root filesystem (SchFS), PCI/USB/virtio drivers, a TTF
rasteriser, and a Linux-numbered syscall ABI.

No history before this point -- the tree was never under version control.
This commit exists so the rename that follows has something to undo to.

kernel/sys/linux_abi.c

@@ -0,0 +1,33 @@
+// SPDX-License-Identifier: Apache-2.0
+// Linux x86_64 syscall number -> Schunix syscall number translation.
+// Enabled per process by sys_enable_linux_abi (see kernel/sys/sys_compat.c).
+//
+// This file used to be the second half of a two-place registration scheme: a
+// hand-written `#define LINUX_foo N` for every Linux number, plus a hand-written
+// map entry pointing it at SYS_foo. Adding a syscall meant three edits across
+// two files, and forgetting this one left the syscall unreachable from every
+// musl-linked program while looking perfectly registered in table.inc.
+//
+// The map is now generated from kernel/sys/table.inc, which carries the Linux
+// number alongside the native one. There is nothing to hand-edit here.
+
+#include "types.h"
+#include "syscall.h"
+
+#define LINUX_NSYSCALL NSYSCALLS
+
+// Index: Linux x86_64 syscall number. Value: Schunix SYS_* number, or 0 for
+// "not implemented" (syscall() then returns -ENOSYS).
+//
+// Syscalls declared NO_LINUX in table.inc all land on the single scratch index
+// NO_LINUX. They overwrite each other there, which is harmless: no real Linux
+// syscall uses that number, so the slot is never dispatched.
+int linux_syscall_map[LINUX_NSYSCALL] = {
+#define SYSCALL(nr, lnr, name)   [lnr] = SYS_##name,
+#define SYSCALL_ALIAS(lnr, name) [lnr] = SYS_##name,
+#include "table.inc"
+#undef SYSCALL
+#undef SYSCALL_ALIAS
+};
+
+int linux_syscall_map_size = LINUX_NSYSCALL;