Change -x/--xdev/--one-filesystem to include the mount point itself.

Match rsync, tar, etc., and add and use t/cleanup-mounts-under to
clean up all temporary mounts under t/mnt and t/tmp.

Thanks to Karl Kiniger <karl.kiniger@med.ge.com> for the report.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>