Replace /usr/src with SRC_BASE
Abstract cpdup call into do_clone()
clonefs really has no business mounting /usr/src, move to do_jail_mounts
Only mount /usr/src if it exists
jail: Don't check KERNEL OSVERSION to see if fmake is needed, check userland - poudriere may be jailed
jail -cu: Add -m src=/path/to/src method to use pre-existing src|obj dir