pfctl: Optionally show gateway information for states

When showing the states, in very verbose mode, also display the gateway
(i.e. the target for route-to/reply-to).

Submitted by: Steven Brown
Reviewed by: donner
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D30051