First pass.

Move igb(4) out of sys/dev/e1000 with its own copy of "shared code"

Remove the igb(4) related shared code files from sys/dev/e1000

Remove a coupld of em/lem conditionals in various switch/case statements
from igb(4) code that shouldn't be there in the first place.

Literally rename all symbols in igb(4) via s/e1000_/igb_/g that are global.
I think I got all of them, but needs review.