Save up to 6 seconds during slave destruction

The previous code blindly tried to umount any possible mount, twice,
with 3 seconds of sleep before attempts.  The new code founds out what
is actually mounted and only attempts to umount those in a single
umount instance.  Additionally, if successful, the 3-second wait is
skipped.  This saves 6 seconds and a lot of known failed umount attempts.
Fix ccache handling and ctrl-t label
Signficantly beef up mtree exclude lists

DragonFly has readonly worlds, so we can exclude everything except the
tmpfs mounts (and even some of those we can exclude).  This should speed
up the mtree steps signficantly.
Fix mounting bug when stopping all builders
The equivalent of find * -print -quit is find * -print | head -1

This fix unbreaks the build again.