add if_rxr_livelocked so rxr users can request backpressure themselves.

right now the rx ring moderation code makes a decision globally
that a machine is livelocked, and uses that to apply backpressure
on all the rx rings. we're moving toward having the network stack
run on multiple cpus, and fed from multiple rx rings. if_rxr_livelocked
lets a driver apply backpressure explicitely if something tells it
that whatever is consuming previous packets cannot keep up.

while here expose the current ring watermark with if_rxr_cwm.

tweaks and ok visa@

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