Updates to mxge for multiple tx/rx rings:

• Update mxge to use if_transmit(), and the new buf_ring interfaces, so as to enable multiple transmit queues. Use of if_transmit() is conditional on IFNET_BUF_RING, and is enabled by default (as in if_em).

• Record a flow id on receive if receive hashing is active. I currently only record the rx ring id (0..8) rather than the 32-bit topelitz hash result, as doing the latter would require shifting the driver to use a larger rx return ring.

Sponsored by: Myricom, Inc.