Re-introduce TSO support after we've implemented fixes for the two reported

Re-introduce TSO support after we've implemented fixes for the two reported
issues:

1. Unaligned memory access panic on sparc64 -> Made ether_extract_headers()
   memory alignment safe.
2. em(4) watchdog timeouts in conjunction with ix(4)/vlan(4) -> Fixed
   RX/LRO packet size calculation used for TSO tagging in ix(4).

Extensive testing done by bluhm@ on amd64 and sparc64 based on different
chips.
Testing done on Hrvoje Popovskis ix(4)/em(4)/vlan(4) setup from where the
issue 2 was reported.

OK bluhm@