scanimage: mention when an option is advanced

The --all-options to scanimage is ambiguous - it toggles the display of options without SANE_CAP_SOFT_SELECT, but I had always read it as having something to do with the SANE_CAP_ADVANCED flag. This commit prints [advanced] next to advanced options, which resolves the ambiguity in a way that hopefully won't break existing apps or require extra translation work.

I'm submitting this as a merge request because it seems like a simple solution. Feel free to treat this as a feature request if you have a better idea :)