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.

include/syscall.h

@@ -0,0 +1,49 @@
+#pragma once
+
+// Syscall numbers and definition macros — all derived from
+// kernel/sys/table.inc, which is the single source of truth.
+//
+// Do NOT add SYS_* constants by hand, and do NOT add anything to
+// kernel/sys/linux_abi.c: add one SYSCALL() line to table.inc and write the
+// body with SYSCALL_DEFINE(). The constant, the dispatch slot and the Linux
+// ABI mapping are all generated from that one line.
+
+#define NSYSCALLS 512
+
+// Scratch slot for syscalls with no Linux equivalent. Every such entry lands
+// on this one index of the translation map, which is never dispatched (Linux
+// x86_64 numbers currently top out around 450). Using a real index rather
+// than a negative sentinel keeps the generated map a plain designated
+// initialiser instead of needing a second macro.
+#define NO_LINUX 511
+
+#ifdef __ASSEMBLER__
+// In .S files GCC passes through the C preprocessor but the assembler
+// sees the output, so emit .set directives instead of an enum.
+#define SYSCALL(nr, lnr, name) .set SYS_##name, nr
+#define SYSCALL_ALIAS(lnr, name)
+#include "../kernel/sys/table.inc"
+#undef SYSCALL
+#undef SYSCALL_ALIAS
+#else
+// In C files generate a proper enum so constants work in switch/designators.
+enum syscall_nr {
+    _SYS_RESERVED = 0,
+#define SYSCALL(nr, lnr, name) SYS_##name = (nr),
+#define SYSCALL_ALIAS(lnr, name)
+#include "../kernel/sys/table.inc"
+#undef SYSCALL
+#undef SYSCALL_ALIAS
+};
+
+// Every syscall body is written as:
+//
+//     SYSCALL_DEFINE(getpid)
+//     {
+//         return myproc()->pid;
+//     }
+//
+// so the signature lives in exactly one place and every implementation is
+// findable with a single grep.
+#define SYSCALL_DEFINE(name) addr_t sys_##name(void)
+#endif