Add comments about needing flow details updated here and there.
Add a new function - rss_gethashconfig() - that returns what the currently
configured hashing looks like.

It's intended for NIC drivers to configure their hash setups.
Use the new API to grab the hash config.

Don't hash the default setup to 4-tuple UDP, at least until fragment
handling is fixed.
Flip back to 2-tuple for UDP for now and document why.