Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Add alc(4), a driver for Atheros AR8131/AR8132 PCIe ethernet
controller. These controllers are also known as L1C(AR8131) and
L2C(AR8132) respectively. These controllers resembles the first
generation controller L1 but usage of different descriptor format
and new register mappings over L1 register space requires a new
driver. There are a couple of registers I still don't understand
but the driver seems to have no critical issues for performance and
stability. Currently alc(4) supports the following hardware
features.
o MSI
o TCP Segmentation offload
o Hardware VLAN tag insertion/stripping
o Tx/Rx interrupt moderation
o Hardware statistics counters(dev.alc.%d.stats)
o Jumbo frame
o WOL
AR8131/AR8132 also supports Tx checksum offloading but I disabled
it due to stability issues. I'm not sure this comes from broken
sample boards or hardware bugs. If you know your controller works
without problems you can still enable it. The controller has a
silicon bug for Rx checksum offloading, so the feature was not
implemented.
I'd like to say big thanks to Atheros. Atheros kindly sent sample
boards to me and answered several questions I had.

HW donated by: Atheros Communications, Inc.