hyperv/hn: Implement transparent mode network VF.

[sepherosa_gmail.com]

Summary

How network VF works with hn(4) on Hyper-V in transparent mode:

- Each network VF has a cooresponding hn(4).
- The network VF and the it's cooresponding hn(4) have the same hardware
  address.
- Once the network VF is attached, the cooresponding hn(4) waits several
  seconds to make sure that the network VF attach routing completes, then:
  o  Set the intersection of the network VF's if_capabilities and the
     cooresponding hn(4)'s if_capabilities to the cooresponding hn(4)'s
     if_capabilities.  And adjust the cooresponding hn(4) if_capable and
     if_hwassist accordingly. (*)
  o  Make sure that the cooresponding hn(4)'s TSO parameters meet the
     constraints posed by both the network VF and the cooresponding hn(4).
     (*)
  o  The network VF's if_input is overridden.  The overriding if_input
     changes the input packet's rcvif to the cooreponding hn(4).  The
     network layers are tricked into thinking that all packets are
     neceived by the cooresponding hn(4).
  o  If the cooresponding hn(4) was brought up, bring up the network VF.
     The transmission dispatched to the cooresponding hn(4) are
     redispatched to the network VF.
  o  Bringing down the cooresponding hn(4) also brings down the network
     VF.
  o  All IOCTLs issued to the cooresponding hn(4) are pass-through'ed to
     the network VF; the cooresponding hn(4) changes its internal state
     if necessary.
  o  The media status of the cooresponding hn(4) solely relies on the
     network VF.
- Once the network VF is detached.  Undo all damages did to the
  cooresponding hn(4) in the above item.

NOTE:
No operation should be issued directly to the network VF, if the
network VF transparent mode is enabled.  The network VF transparent mode
can be enabled by setting tunable hw.hn.vf_transparent to 1.  The network
VF transparent mode is _not_ enabled by default, as of this commit.

The benefit of the network VF transparent mode is that the network VF
attachment and detachment are transparent to all network layers; e.g. live
migration detaches and reattaches the network VF.

The major drawbacks of the network VF transparent mode:
- The netmap(4) support is lost, even if the VF supports it.
- ALTQ does not work, since if_start method cannot be properly supported.

(*)
These decisions were made so that things will not be messed up too much
during the transition period.