e1000: lem(4)/em(4) ifcaps, TSO and hwcsum fixes

• em(4) obey administrative ifcaps for using hwcsum offload
• em(4) obey administrative ifcaps for hw vlan receive tagging
• em(4) add additional TSO6 ifcap, but disabled by default as is TSO4
• lem(4) obey administrative ifcaps for using hwcsum offload
• lem(4) add support for hw vlan receive tagging
• lem(4) Add ifcaps for TSO offload experimentation, but disabled by default due to errata and possibly missing txrx code.
• lem(4) disable HWCSUM ifcaps by default on 82547 due to errata around full duplex links. It may still be administratively enabled.

Reviewed by: markj (previous version)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D30072