MFT: jstop: Always kill -9 -1 in the jail before calling jail -r

jail_remove(2) does SIGKILL all jailed processes, but for some reason
still has file descriptors open to the jail after the call.
Even adding a sleep(1) after the jail -r does not help.

The issue this is fixing is easily recreatable by building devel/libtool
and hitting ctrl+c in the configure phase. It leaves the builder jail running
and devfs mounted.

^C====>> Error: Signal caught, cleaning up and exiting
====>> Cleaning up
umount: unmount of /usr/local/poudriere/data/build/91i386-default/01/dev failed: Device busy
====>> Umounting file systems

   JID  IP Address      Hostname                      Path
   130  127.0.0.1       91i386-default-job-01         /usr/local/poudriere/data/build/91i386-default/01

With this change, the issue goes away.