destroy_slave: Use current version of directories

Need to follow poudriere => .p and newpackages => .npkg change of
create_slave on destroy_slave.

Also remove unused killtree procedure
Attempt to improve dismounting

Call umount once per mount rather than as a list of arguments.  Also,
never use "-f" with umount (from experience, this is a bad idea).
Hopefully this fixes some unclean aborts and other randomly strange
observations.