802.11w / MFP (management frame protection) - also called PMF
(protected management frames) - requires two more key slots
for the broadcast keys. The broadcast management traffic
isn't encrypted - instead each frame has an MMIC IE at the
end which verifies the frame.

This commit extends the number of keys from 4 to 6 and
adds separate functions for the IGTK key type and index.

Notably the 802.11 frame format only allows four key IDs
(0..3) and unicast management traffic is just normally
encrypted - the broadcast frames will have the actual key
index (4, 5) in the MMIC IE.