Eliminate proto_tree_add_text from some of the dissectors.

Other minor cleanups while in the area.

Change-Id: I8ea59205cfe6fab643d8fe01b75ce91532004fd9
Reviewed-on: https://code.wireshark.org/review/4004
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: Alexis La Goutte <alexis.lagoutte@gmail.com>