Releng 2 1 Enhance interface gateway data entry descriptions

There have been a lot of people recently on the forum who have felt the urge to enter a gateway on LAN. So this is my suggestion for changing the wording of the prompts and GUI to try to help people to make the right decision. I am looking for comments on whether this seems a reasonable way to go, and if there are other suggestions for the wording. We do not want to fill the user interface with too many words, but we do need to get the message across.
This code is based in 2.1 branch, but I can easily make a pull request for master when there is agreement about the wording.
Or, feel free to commit this if you like it, and merge it forward to master or...