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

Summary:
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.

Sponsored-by: Rubicon Communications, LLC ("Netgate")
Reviewed-by: philip, adrian
MFC-after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D25999