i386: Make boot loader smaller by reducing size of bootinfo

We don't need the 56 bytes at the end of bootinfo, and never had. Don't
copy them from old boot loaders, and don't provide them with new boot
loaders.

Add comments about what versions of FreeBSD 'old' means in various
contexts. Add note that old disk loader (from 1.x/2.x) is doomed to
failure because it doesn't provide metadata that we now require to boot,
and has been since approximately FreeBSD 7.x. Retain all this
information to explain why we have 4 arguments that are always 0, even
though it's ancient history.

This saves 56 bytes in the boot loader.

Sponsored by: Netflix
Reviewed by: phk, rgrimes, kib
Differential Revision: https://reviews.freebsd.org/D36550