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.

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

Maybe the string boot shouldn't appear in xv6 code?

Allocate proper kernel page table immediately in main using boot allocator
Remove pginit
Simplify address space layout a tiny bit
More to come (e.g., superpages to simplify static table)

Very important to give qemu memory through PHYSTOP :(