Checkpoint switching to per-process locks, in attempt clarify xv6's

locking plan, which is a difficult to understand because ptable lock
protects many invariants.  This implementation has a bug: once in a
while xv6 unlocks a proc lock that is locked by another core.

kernel/sysproc.c

@@ -4,6 +4,7 @@
 #include "date.h"
 #include "param.h"
 #include "memlayout.h"
+#include "spinlock.h"
 #include "proc.h"
 
 int