Eliminate proto_tree_add_text from some of the dissectors.

Other minor cleanups while in the area.

Change-Id: Id8dab02df6f111c3462238c3de1bf201e037ca48
Reviewed-on: https://code.wireshark.org/review/4022
Reviewed-by: Michael Mann <mmann78@netscape.net>
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Evan Huus <eapache@gmail.com>