Sync with Intel's em(4) driver version 3.2.15

Sync with Intel's em(4) driver version 3.2.15
- Add support for 82571EB, 82572EI and 82573E adapters
- Nuke DBG_STATS related code and struct adapter's fields
- Flow control water marks are now calculated from hardware's register value
  instead of using hardwired values(FC_DEFAULT_{LOW, HIGH}_THRESH)
- Flow control pause time is bumped to 512us (old value is 131us) (*)
- In em_detach(), call bus_teardown_intr() if adapter.int_handler_tag != NULL
- Update Intel's LICENSE and README
- Update README to reflect recent changes to device polling
- Minor style changes

(*) Original code misuses 0x1000 (should be 1000) as em_hw.fc_pause_time