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>