Eliminate proto_tree_add_text from some of the dissectors.

Other minor cleanups while in the area.

Change-Id: I623d941e53128f169e55dfc629547b4221fa72fc
Reviewed-on: https://code.wireshark.org/review/4021
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>