tohex: Add -d/--decimal option

3ebd4bc4-dd19-11e2-955c-0800274013ad
stpl: Remove "set encoding iso_8859_1", don't use it these days

It's been there for almost ten years, time to enter the new millenium
properly.

ddbce7f0-dda2-11e2-b6a5-0800274013ad
stpl: Don't use "set terminal x11"

It's kind of... how shall I put it... ugly? It's been there from the
beginning, had probably a reason for using it. It was 2003, after all.

8304fe6e-dda3-11e2-8372-0800274013ad