MFC r301584: Apply mergemaster r255428:

Pass -n (do not emit comments) when saving mtree information for future
mergemaster(8) runs.

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

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.

Add a script to create the /var/db/mergemaster.mtree file for new
releases so that when users subsequently update their source trees
they can make use of mergemaster's -U option.