Fixes and enhancements for the if_cgem driver...

• miibus fixes as suggested by Yonghyeon Pyun.
• enable VLAN MTU support.
• fix a few WITNESS complaints in cgem_attach().
• have cgem_attach() properly init the ifnet struct before calling mii_attach() to fix panic when using e1000phy.
• fix ethernet address changing.
• fix transmit queue overflow handling.
• tweak receive queue handling to reduce receive overflows.
• bring out MAC statistic counters to sysctls.
• add e1000phy to config file.
• implement receive hang work-around described in reference guide.
• change device name from if_cgem to cgem to be consistent with other interfaces.

Submitted by: Thomas Skibo <ThomasSkibo@sbcglobal.net>
Reviewed by: wkoszek, Yonghyeon PYUN <pyunyh@gmail.com>