teach libkvm not to assume /dev/mem
This patch brings in a couple of trivial changes from freebsd that allow me to
use kgdb on /dev/fwmem0.0 (unfortunately, when using fwmem kgdb returns
nonsensical values for some variables, expected values for others and fails
to access some valid addresses). It also fixes what I think is an obvious
typo (vmfd -> pmfd).