Fully synchronize sys/boot from FreeBSD-5.x, but add / to the module path

Fully synchronize sys/boot from FreeBSD-5.x, but add / to the module path
so /kernel will be found and loaded instead of /boot/kernel.  This will
give us all the capabilities of the FreeBSD-5 boot code including AMD64 and
ELF64 support.

This commit brings userland up to date with the kernel and boot code changes,
primarily due to sys/diskmbr.h separation from sys/disklabel.h, and
linker_set changes.