Add support to easily build FreeBSD unpacked in a chroot of another

Add support to easily build FreeBSD unpacked in a chroot of another
FreeBSD machine.  To do this add the man 1 uname changes to __xuname.c
so we can override the settings it reports.  Add OSVERSION override
to getosreldate.  Finally which Makefile.inc1 to use uname -m instead
of  sysctl -n hw.machine_arch to get the arch. type.

With these change you can put a complete FreeBSD OS image into a
chroot set:
	UNAME_s=FreeBSD
	UNAME_r=4.7-RELEASE
	UNAME_v="FreeBSD $UNAME_r #1: Fri Jul 22 20:32:52 PDT 2005 fake@fake:/usr/obj/usr/src/sys/FAKE"
	UNAME_m=i386
	UNAME_p=i386
	OSVERSION=470000
on an amd64 or i386 and it just work including building ports and using
pkg_add -r etc.  The caveat for this example is that these patches
have to be applied to FreeBSD 4.7 and the uname(1) changes need to
be merged.  This also addresses issue with libtool.

This is usefull for when a build machine has been trashed for an
old release and we want to do a build on a new machine that FreeBSD
4.7 won't run on ...