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.

lib/start.S

@@ -0,0 +1,44 @@
+/* SPDX-License-Identifier: Apache-2.0
+ * Schunix — process entry point.
+ *
+ * This has to be assembly, and the reason is worth recording.
+ *
+ * exec() hands a new process its arguments in registers, not on the stack:
+ * kernel/fs/exec.c sets proc->tf->rdi = argc and proc->tf->rsi = argv. Those
+ * are already the first two SysV argument registers, so _start's entire job
+ * is to call main and then exit — it must not touch %rdi or %rsi at all.
+ *
+ * It used to be written in C, capturing the incoming registers with GCC's
+ *
+ *     register int argc_reg asm("rdi");
+ *
+ * That is not what local register variables mean. Both GCC and clang only
+ * guarantee the binding when the variable is used as an operand of an inline
+ * asm statement; reading one to pick up an incoming argument is undefined.
+ * GCC happened to do the useful thing. clang compiled it to
+ *
+ *     movl  -0x4(%rbp), %eax        # two uninitialised stack slots
+ *     movq  -0x10(%rbp), %rax
+ *
+ * and every program was handed a garbage argc and argv. Nothing crashed --
+ * init, shmtest and socktest take no arguments and passed their whole test
+ * suites -- so the only symptom was `inputtest -n 6` quietly ignoring its
+ * limit. A compiler-dependent way to read registers has no business being in
+ * C when four instructions of assembly are exactly right.
+ */
+
+.text
+.globl _start
+_start:
+    xorq    %rbp, %rbp          /* end the frame-pointer chain here */
+
+    /* SysV wants %rsp 16-byte aligned immediately before a call, so that the
+     * callee sees %rsp+8 aligned once the return address is pushed. Rounding
+     * down is safe: argc and argv arrive in registers, so nothing below is
+     * needed. */
+    andq    $-16, %rsp
+
+    callq   main                /* main(%rdi = argc, %rsi = argv) */
+    callq   exit                /* exit() is noreturn, but be certain */
+1:  hlt
+    jmp     1b