Unmount filesystems with the "-n" option so no "sync" is done on
the unmount. We are destroying the main filesystem immediately
afterward anyway, and the /dev and /dev/fd instances are likewise
disposable (the lib/geni directory itself is mounted RO).

Up the allowable limit on open files. Apparently we were hitting
this in a couple of places when going through the /etc/rc.d scripts
and each time resulted in a half second pause.

These two tweaks get us from 6+ seconds per jail invocation to
under 2 seconds. I think even on a non-trivial script, the savings
will be meaningful.