Refuse to destroy a jail with mounted filesystems

```
 # 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
```

usr/local/share/bastille/destroy.sh

@@ -69,6 +69,11 @@ destroy_jail() {
         fi
 
         if [ -d "${bastille_jail_base}" ]; then
+            ## make sure no filesystem is currently mounted in the jail directory
+            mount_points=$(mount | cut -d ' ' -f 3 | grep "${bastille_jail_base}")
+            if [ $? -eq 0 ]; then
+                error_exit "Jail has mounted filesystems:\n$mount_points"
+            fi
             ## removing all flags
             chflags -R noschg "${bastille_jail_base}"