Mainly delete unused constants and code.

Move mp_startthem to main.c as bootothers.

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

working unlink, but doesn't free dir blocks that become empty
remove out-of-date comment in ioapic

give cpu1 a TSS and gdt for when it enters scheduler()
and a pseudo proc[] entry for each cpu
cpu0 waits for each other cpu to start up
read() for files

ioapic