pf: support source-hash and random with tables and dynifs, not just pools

This finally allows to use source-hash for dynamic loadbalancing, eg.
"rdr-to <hosts> source-hash", instead of just round-robin and least-states.

An older pre-siphash version of this diff was tested by many people.

OK tedu@ benno@

Obtained from: OpenBSD, reyk <reyk@openbsd.org>, 252a05523f
Sponsored by: Rubicon Communications, LLC ("Netgate")