- em(4) igb(4) and lem(4)
- deprecate the igb device from kernel configurations
- create a symbolic link in /boot/kernel from if_em.ko to if_igb.ko

Devices tested:
- 82574L
- I218-LM
- 82546GB
- 82579LM
- I350
- I217

Please report problems to freebsd-net@freebsd.org

Partial review from jhb and suggestions on how to *not* brick folks who
originally would have lost their igbX device.

Submitted by:	mmacy@nextbsd.org
MFC after:	2 weeks
Relnotes:	yes
Sponsored by:	Limelight Networks and Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D8299