Refuse to destroy a jail with mounted filesystems

[gahr]

# bastille create temp 14.0-RELEASE 192.168.1.10 lo1
...
# mkdir test
# cp /usr/bin/less test/
# mkdir /usr/local/bastille/jails/temp/root/test
# bastille mount temp $(realpath test) test
[temp]:
Added: /root/admin/bastille/test /usr/local/bastille/jails/temp/root/test nullfs ro 0 0
# /usr/local/bastille/jails/temp/root/test/less -f /dev/stdin &
# bastille destroy force temp
rdr-anchor not found in pf.conf
[temp]:
temp: removed
umount: unmount of /usr/local/bastille/jails/temp/root/test failed: Device busy
jail: temp: /sbin/umount -t nullfs /usr/local/bastille/jails/temp/root/test: failed

Deleting Jail: temp.
Jail has mounted filesystems:
/usr/local/bastille/jails/temp/root/test

Fixes #662