options: change option parsing when using a single dash

Addresses dumb things like accidentally overwriting a media file with
e.g. "mpv --log-file test.mkv" (when the user thought that --log-file
was a flag option, when it actually takes a filename). This example will
now print an error. It still works with "-log-file overwritten.mkv", but
prints a warning.

Not sure if I'm being too careful or not "radical" enough. In any case,
both the syntax that stops working and the syntax that produces a
warning now have been discouraged and were called legacy for almost a
decade.