pf: print rule number in debug output

When printing states in debug output, print the rule number that created
it if we have it.

Requested by dlg, ok henning.

Obtained from: OpenBSD, mcbride <mcbride@openbsd.org>, c06d1661a6
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D46585