Eliminate proto_tree_add_text from some dissectors.

Other minor cleanup while in the neighborhood.

Change-Id: If73a029f564219782c313d4154c24c7ce7458b52
Reviewed-on: https://code.wireshark.org/review/3574
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: Anders Broman <a.broman58@gmail.com>