Rewrite the ~/.xscreensaver parser to match XScreenSaver's exactly.

Behavioral changes/fixes:

- Whitespace skipping now is performed in all appropriate places.
- Visual skipping now happens on the same set of visual names.
- Quoted arguments in commands now work. In fact, so do all other shell
  characters.

One remaining difference though (intentional):

- The program name itself can't be quoted.