MFC r314830:
mergemaster: fix description of -p
-p only handles updating /etc/master.passwd and /etc/group . No more,
no less.
Also, mergemaster (and no other portions of the vanilla FreeBSD build
process) should be messing with __MAKECONF or SRCCONF as part of the
installworld or distribution process. Don't insinuate that mergemaster
does that as it's a false claim.