2017 IFLIB updates in preparation for commits to e1000 and ixgbe.

2017 IFLIB updates in preparation for commits to e1000 and ixgbe.
- iflib - add checksum in place support (mmacy)
- iflib - initialize IP for TSO (going to be needed for e1000) (mmacy)
- iflib - move isc_txrx from shared context to softc context (mmacy)
- iflib - Normalize checks in TXQ drainage. (shurd)
- iflib - Fix queue capping checks (mmacy)
- iflib - Fix invalid assert, em can need 2 sentinels (mmacy)
- iflib - let the driver determine what capabilities are set and what
          tx csum flags are used (mmacy)
- add INVARIANTS debugging hooks to gtaskqueue enqueue (mmacy)
- update bnxt(4) to support the changes to iflib (shurd)

Some other various, sundry updates.  Slightly more verbose changelog:

Submitted by:	mmacy@nextbsd.org
Reviewed by:	shurd
mFC after:
Sponsored by:	LimeLight Networks and Dell EMC Isilon