if no lapic available, use 8253pit for clock
now xv6 runs both on qemu (uniprocessor) and bochs (uniprocessor and MP)

bug in proc_wait

The use/defs extraction in runoff still needs some work
for xv6 code style.

xv6 seems to work fine on uniprocessor now.

don't use the same name for two different pieces of code