This way, the bootloader doesn't have to translate the entry point.
This also makes xv6 multiboot-compliant and follows the convention
used by Linux.

Previously, deallocuvm scanned from 0 to KERNBASE in one page
increments, which had a noticable effect on boot time.  Now it skips
over missing page directories.

Conflicts:
	vm.c

exec() with arguments that don't fit on a single page.