TOOLS/umpv: allow passing 0 files

Some might use that to just create an empty window with --force-window
--idle (for whatever reasons).
TOOLS/umpv, mpv.desktop: use --no-terminal instead of --really-quiet

So this won't initialize terminal handling if stdout happens to be a
terminal. It also suppresses all output to stdout/stderr.
TOOLS/umpv: don't mangle URLs

This attempted to prefix the current directory to URLs, because it
didn't recognize them as already absolute paths.