Sync with Intel's em(4) driver version 6.1.4

Sync with Intel's em(4) driver version 6.1.4
- Add support for new chipset revisions embedded in the ESB2 and ICH8 core
  logic chipsets.
- Add PCI id for the PCIe quad port copper adapter.	[*]
- Improve media support for fiber adapters.		[*]
- Instead of creating TX DMA map in em_encap(), create all of the during
  em_init().
  Fix a bug in Intel's code:
  The TX desc used to hold mbuf, does not necessarily have the DMA map
  that is used to load the mbuf, so in em_clean_transmit_interrupts() or
  during em_stop(), wrong DMA map may be used.
- Update README

Bug fixes:
- Properly serialize em_shutdown()
- (BUS_DMASYNC_PREWRITE | BUS_DMASYNC_PREREAD) -> BUS_DMASYNC_PREWRITE
- Correct polling(4) support: disable interrupt if polling(4) is enabled.

Tested with an 82540EM.

# I have committed a slightly different version(6.1.4) than the version(6.0.5)
# posted to submit@, since 6.1.4 came immediately after my submission for
# testing/reviewing, and the difference between it and the submitted version
# is really very small (only added [*] part).