Parse external ACL key-pair in HelperReply

Parse external ACL key-pair in HelperReply

Moves parsing of tag, user, password, message, and log key-pair to shared
helper parser. Enabling these keys to be implemented by other helpers.

Adds deprecation/unsupported warning to external ACL parser for other
key-pair.

TODO: support unknown key=pair in generic parser. At present the first
      unknown key will halt parsing of the whole line.