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/glue.c

@@ -0,0 +1,24 @@
+// SPDX-License-Identifier: Apache-2.0
+// Schunix — first-party userland runtime: _start entry and exit.
+
+#include "types.h"
+#include "user.h"
+
+extern void _xv6_exit(void);
+
+void
+exit(void)
+{
+    _xv6_exit();
+    for (;;);
+}
+
+/* _start lives in lib/start.S — it must read incoming registers, which
+ * cannot be expressed portably in C. See that file. */
+
+void
+__stack_chk_fail(void)
+{
+    _xv6_exit();
+    for (;;);
+}