Expose system level ixgbe sysctls.
Device level sysctls are already exposed as dev.ix.<device>

Fixing the case where number of queues for igb is auto-tuned and
hw.igb.num_queues does not return current/updated value.

Reviewed by:	jfv
Approved by:	re (delphij)
MFC after:	2 weeks

(cherry picked from commit e6885256cdc6759add16ed571a19768419ac65ef)
Update the Intel igb driver to version 2.4.0
  - This version has support for the new Intel Avoton systems,
including 2.5Gb support, further it now has IPv6/TSO6 support as
well. Shared code has been updated where necessary as well. Thanks
to my new assistant Eric Joyner for doing the transmit path changes
to bring in the IPv6/TSO6 support. Thanks to Gleb for catching the
one bug and change needed in NETMAP.

Approved by: re

(cherry picked from commit 7f364f2263dcbf5399bd43a3676145dd57c6613b)

Ticket: #3589