Bug 9586 says bit 0x00000100 in the "802.11n flags" means "802.11ac".

Also, the 0x00000100 flag is "MCS index used", if a dissection of an
802.11n frame by OmniPeek is to be believed and if the "802.11n flags"
have the same meaning in Peek tagged files and in the Peek remote
protocol.

Rename that field to "extended flags", as it's not just for 802.11n, add
the "802.11ac" flag, and rename the "future use" flag.

Change-Id: I605622801450d5d114c3c971c98960b198346bdd
Reviewed-on: https://code.wireshark.org/review/4968
Reviewed-by: Guy Harris <guy@alum.mit.edu>