(split) Set the back pointer from the prefix to the interface before adding
the prefix to the interface's prefix list.  This shouldn't make a
difference, since rtadvd(8) is single-threaded, but I've seen it crash
in delete_prefix() with pfx_rainfo == NULL, and this is the only place
where a prefix can be added to the list with a NULL pfx_rainfo.

MFC after:	3 days
(split) Migrate the ff_encap1() routine out into the normal output code.

This will eventually be used by the A-MSDU encapsulation code that
I'm writing - the sub-frame encapsulation requirement is the same.
(split) Create a new function to complete 802.11 mbuf transmission.

The aim of this function is to eventually be the completion entry point
for all 802.11 encapsulated mbufs.  All the wifi drivers end up doing
what is in this function so it's an easy win to turn it into a net80211
method and abstract out this code.

Ideally the drivers will all eventually be modified to queue up completed
mbufs and call this function with all the driver locks not held.
This will allow for some much more interesting software queue handling
in the future (like net80211 based A-MSDU, fast-frames, A-MPDU aggregation
and retransmission.)

Tested:

* ath(4), iwn(4)
(split) Merge branch '1d2edd95'