IEEE 802.11w/D6.0 defines new AKMPs to indicate SHA256-based algorithms for
key derivation (and AES-CMAC for EAPOL-Key MIC). Add support for using new
AKMPs and clean up AKMP processing with helper functions in defs.h.
This gets rid of potential warnings about buffer bounds errors. The earlier
code works fine, but it is not the cleanest way of using the struct wpa_ptk
definition for TK1/TK2.