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/spinlock.h

@@ -0,0 +1,9 @@
+// Mutual exclusion lock.
+struct spinlock {
+  uint64 locked;      // Is the lock held?
+  // For debugging:
+  char *name;        // Name of lock.
+  struct cpu *cpu;   // The cpu holding the lock.
+  addr_t pcs[10];    // The call stack (an array of program counters)
+                     // that locked the lock.
+};