Copy em(4) to emx(4), which supports only 8257{1,2,3} parts.

Copy em(4) to emx(4), which supports only 8257{1,2,3} parts.

The reasons to create another driver for 8257{1,2,3}:
- Various old hardware bug workaround code are removed, so the
  code is more straightforward, especially on the transmit path.
- Only 8257{1,2,3} support multi RX queues.
- Only 8257{1,2} support multi TX queues (no plan for it yet).
- It could be a sandbox for me to add multi queues support, while
  em(4) always works :).