release: Add support for building on non-FreeBSD

This requires two sets of changes. Firstly, for non-FreeBSD, we do not
know where tools are in PATH (and it is likely that some are not in
system directories and have been built as bootstrap tools during the
build), so we should leave PATH alone and trust the user. Secondly,
makefs needs a master.passwd for building images from a METALOG file, so
pass the directory in the image tree to makefs's -N option in order to
pick up a valid FreeBSD master.passwd; this is unnecessary on FreeBSD
(except in the edge case of building an image that refers to users or
groups not present in the host's database, which is unlikely but
technically possible) but harmless so can be done unconditionally.

Reviewed by: brooks, emaste, gjb
Differential Revision: https://reviews.freebsd.org/D34001