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.

As part of this work, rather then try to adjust ufs/fs.h and friends to get
UFS2 info I instead copied the fs.h and friends from FreeBSD-5 into the
sys/boot subtree

Additionally, import Peter Wemm's linker set improvements from FreeBSD-5.x.
They happen to be compatible with GCC 2.95.x and it allows very few changes
to be made to the boot code.

Additionally import a number of other elements from FreeBSD-5 including
sys/diskmbr.h separation.