Required e.g. for feh --preload --thumbnails

* remove unused variable Im1 in filelist.c
* options are initialised as 0, so only explicitly set those to 0 for which it
  makes sense because of their context
* Make a few warnings about wrong option combinations fatal

Note that read_filelist used to check if its argument was an image (for people
accidentaly typing "feh -df *" instead of "feh -dF *"); this was removed
because it failed on /dev/stdin.

I might re-add it, but I'm not yet sure about the future of feh's automatic
filelist saving.

  than the filesystem's (possibly random) order.

This fixes imlib2 warnings (and occasional X errors/segfaults), which used to
occur when all files in feh's filelist were suddenly deleted by an external
application. Now, feh simply quits with "No more slides in show" (which makes
more sense, IMHO).