Fix 'make release' on older hosts: use buildworld legacy utilities.

Newer FreeBSD installs require an install(1) that supports the new flags.

This adds ${MAKEOBJDIRPREFIX}${.CURDIR}/tmp/legacy/{bin,usr/{bin,sbin}}
to the PATH while generating an mtree database for 'make release'.

Note that the problem only exists here because mm-mtree.sh generates
its own object tree to avoid mucking with the existing one, which
results in a PATH containing legacy utility dirs that are empty.