Eliminate proto_tree_add_text from some dissectors.

Other minor cleanup while in the neighborhood.

Change-Id: I77cac916d617f56f92f686e9cd9f15fba058facf
Reviewed-on: https://code.wireshark.org/review/3675
Reviewed-by: Michael Mann <mmann78@netscape.net>
Petri-Dish: Michael Mann <mmann78@netscape.net>
Reviewed-by: Anders Broman <a.broman58@gmail.com>