lib80211: Start adding 11ac ETSI bits to regdomain.xml

This change currently (partially) duplicates AC1 freqbands as AC2
as they are not fully overlapping.
It then adds the 11ac netband to the "etsi" domain including
"indoor" and "dfs" flags, which we can deal with, as well as
appropriate (round down) maxpower values.
Comments are left for the actual frequency bands as we do use the
centerfreq for the first/last (chansep sized) channel in the
freqband and their "id" name, which can be confusing.

Reviewed by: philip, adrian
Differential Revision: https://reviews.freebsd.org/D25999

(cherry picked from commit 2ad93dade70a9c98174fc87d0cd24fd54bd0d120)