Eliminate proto_tree_add_text from packet-ppp.c.

Some of the conversions to proto_tree_add_xxx calls involved using explicit sizes instead of the "option length", so there is skipping of the "byte highlighting" of the type + length bytes.

Change-Id: Ibfe09a089fb33ba43121de079aca81742d48db4c
Reviewed-on: https://code.wireshark.org/review/5636
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: Christopher Maynard <Christopher.Maynard@gtech.com>