mapping of msgbuf during startup may map invalid physical adresses
"If the last available physical memory segment on a system is less 16k,
than the startup code that will map the kernel message buffer, will fail
and map physical pages behind the last segment. This may either only lead
to a message buffer without physical memory behind it, or to an
overlapping message buffer with something else."
Fix by allowing multiple physical memory segments to be used for msgbuf.
Also remove some leftover msgbuf manipulation from pmap.c.
Fix supplied by Wolfgang Stukenbrock in the PR, with some modifications
from me, mainly to use the already existing constant VM_PHYSSEG_MAX as the
static limit of number of msgbuf segments.
9e8014fe50