[net80211] begin fleshing out new hardware crypto offload features.

• extend the keycache flag word to be 32 bits, not 16 bits
• add new key flags for transmit: + IEEE80211_KEY_NOIV: Don't insert IV in the payload when transmitting data frames; + IEEE80211_KEY_NOIVMGT: Don't insert IV in the payload when transmitting MIC frames; + IEEE80211_KEY_NOMIC: Don't insert MIC in the payload when transmitting data frames; + IEEE80211_KEY_NOMICMGT: don't insert MIC in the payload when transmitting management frames.

• teach ieee80211_crypto_demic() about hardware decrypted frames: + if frames are hardware decrypted and the frame has failed MIC, treat it as a michael failure. + if frames are hardware decrypted and the frame has stripped MIC, we can't check the MIC in the payload - we don't have anything to compare it against.

This is only part of the work required to successfully transmit/receive
hardware crypto frames such as the qualcomm atheros 11ac offload chips.

There will be further work in the transmit and receive path before this
can be done by default.

Reviewed by: avos
Differential Revision: https://reviews.freebsd.org/D8364