chanserv: PUBACL changes

1. Require +A for viewing private AKICK information (op-only reason). It
   is still possible to see AKICKed people via /cs flags #channel +b
   with PUBACL set.
2. Require +A for viewing private information using /cs info #channel.
3. Show PUBACL among channel control flags.

ref: shalture/shalture@b456270..d185745