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

[markj]

Summary

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.

[kp]

We should probably document this in pf(4).
And I should see about adding the others that are missing from that too.

[markj]

Mention the sysctl in pf.4 and pf.conf.5.

[ltning-freebsd_anduin.net]

I know I'm listed as a reviewer, but I can't assess the code change here. The gist of it looks good, though. I'd be a lot less worried about enabling the feature with this limiter in place.

[kp]

It may want a man page date update.