Add support for receiving and setting flowtype, flowid and RSS bucket
information as part of recvmsg().

This is primarily used for debugging/verification of the various
processing paths in the IP, PCB and driver layers.

Unfortunately the current implementation of the control message path
results in a ~10% or so drop in UDP frame throughput when it's used.

Differential Revision: https://reviews.freebsd.org/D527
Reviewed by: grehan