When the pf module is not loaded our calls to pfctl will fail with
unhelpful messages.
This change spares the user the pain of decrypting those messages and manually
enabling pf. It also keeps track if pf was loaded by sshuttle and unloads on
exit if that was the case.

Also fixed the case where both ipv4 and ipv6 anchors were added by sshuttle
but the first call of disable would disable pf before the second call had the
chance of cleaning it's anchor.