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.

release/scripts/mm-mtree.sh

@@ -81,6 +81,11 @@ if [ ! -f ${SOURCEDIR}/Makefile.inc1 -a \
 fi
 
 # Setup make to use system files from SOURCEDIR
+objp=${MAKEOBJDIRPREFIX}
+[ -z "${objp}" ] && objp=/usr/obj
+legacydir=${objp}${SOURCEDIR}/tmp/legacy
+legacypath=${legacydir}/usr/sbin:${legacydir}/usr/bin:${legacydir}/bin
+MM_MAKE_ARGS="${MM_MAKE_ARGS} PATH=${legacypath}:${PATH}"
 MM_MAKE="make ${ARCHSTRING} ${MM_MAKE_ARGS} -m ${SOURCEDIR}/share/mk"
 
 delete_temproot () {