Set started_by_sshuttle False after disabling pf

We set it to true when we enable pf, but do not set it back to False
after disabling. When using IPv4 and IPv6 we end up trying to disable
twice which procudes an error while undoing changes in FreeBSD 11.

sshuttle/methods/pf.py

@@ -64,6 +64,7 @@ class Generic(object):
         pfctl('-a %s -F all' % anchor)
         if _pf_context['started_by_sshuttle']:
             pfctl('-d')
+            _pf_context['started_by_sshuttle'] = False
 
     def query_nat(self, family, proto, src_ip, src_port, dst_ip, dst_port):
         [proto, family, src_port, dst_port] = [