kernel/x86_64: Add a bios.c with only bios_sigsearch().

Unlike i386, bios16/bios32 calls are not supported in x86_64, but
searching for ROM signatures is still useful for certain drivers.

We could use it in powernow(4) and also the (upcoming) IPMI driver
uses it.

Taken-from: FreeBSD
trek(6): Fix bounds checking.

The number of sectors was being used to check input for the quadrant,
and NSECTS > NQUADS.

Taken-from:  FreeBSD ports (games/bsdgames)
Reported-by: marino
build.7: Document installworld-force target.