Break out the current 802.11 software scan methods into an indirect table.

In order for drivers to provide an alternate set of scan methods,
these have to finally use an indirection table and all of the calls
in ieee80211_scan.c need to use said table.

For all existing drivers - this is basically a glorified, KBI-breaking
functional no-op.

This is also not the final form - too much functionality is currently
hiding in ieee80211_scan_sw.c that should be in ieee80211_scan.c.
That'll be the target of some follow-up commits.

Note:

• You have to recompile your kernel/drivers after this - the net80211 KBI has changed.
• I'm not yet planning on bumping any versioning - I have a few more things to shuffle around.

Tested:

• urtwn(4) - STA mode
• Intel 7260 in local repo - overriding the methods and table at attach time has the desired effect (ie, all the methods are called, but nothing is ever performed.)