Includes code for TLB shootdown (which actually seems unnecessary for xv6)

move the SYSCALL number up, so does not overlap the IRQ traps.

Delete parallel port output.
Works well with qemu -nographic mode.

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

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