Because the BTX mini-kernel now uses flat memory mode and clients

Because the BTX mini-kernel now uses flat memory mode and clients
are no longer limited to a virtual address space of 16 megabytes,
only mask high two bits of a virtual address.  This allows to load
larger kernels (up to 1 gigabyte).  Not masking addresses at all
was a bad idea on machines with less than >3G of memory -- kernels
are linked at 0xc0xxxxxx, and that would attempt to load a kernel
at above 3G.  By masking only two highest bits we stay within the
safe limits while still allowing to boot larger kernels.

(This is a safer reimplmentation of sys/boot/i386/boot2/boot.2.c
rev. 1.71.)

Prodded by:	jhb
Tested by:	nyan (pc98)