Switch em(4) to the extended RX descriptor format. This matches the
e1000/e1000e split in linux.

Split rxbuffer and txbuffer apart to support the new RX descriptor format
structures. Move rxbuffer manipulation to em_setup_rxdesc() to unify the
new behavior changes.

Add a RSSKEYLEN macro for help in generating the RSSKEY data structures
in the card.

Change em_receive_checksum() to process the new rxdescriptor format
status bit.

MFC after: 2 weeks
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D3447