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

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

Add driver for Silicon Integrated Systems SiS190/191 Fast/Gigabit Ethernet.
This driver was written by Alexander Pohoyda and greatly enhanced
by Nikolay Denev. I don't have these hardwares but this driver was
tested by Nikolay Denev and xclin.

Because SiS didn't release data sheet for this controller, programming
information came from Linux driver and OpenSolaris. Unlike other open
source driver for SiS190/191, sge(4) takes full advantage of TX/RX
checksum offloading and does not require additional copy operation in
RX handler.
The controller seems to have advanced offloading features like VLAN
hardware tag insertion/stripping, TCP segmentation offload(TSO) as
well as jumbo frame support but these features are not available
yet. Special thanks to xclin <xclin<> cs dot nctu dot edu dot tw>
who sent fix for receiving VLAN oversized frames.