pf: Add a sysctl to limit work done for rdr source port rewriting

It was pointed out that the current approach of exhaustively searching
for a free source port might be very time consuming. Limit the amount
of work that we might do before giving up.

Reviewed by: kp
Reported by: Eirik Øverby <ltning-freebsd@anduin.net>
MFC after: 3 months
Sponsored by: Klara, Inc.
Sponsored by: Modirum
Differential Revision: https://reviews.freebsd.org/D46495

(cherry picked from commit 339a1977c32414f3d23733504955245ca6f3802d)