Added allowlegacyconnects and protocol_version

They are commented out so that CFEngine 3.6 is compatible by default to
old installations.

(cherry picked from commit f03522d18d6263f77cdb8dd91d8daf86bd0dff94)
Clarify trustkeysfrom.

(cherry picked from commit d7e5cf03fd674c873d88b7cae5af9fbdb3a2d3ca)