TOOLS/youtube-dl_mpv.sh: disable globbing before expanding $video_url

$video_url can contain a question mark, which can be expanded to a
character in an existing file name if globbing is enabled.
TOOLS/youtube-dl_mpv.sh: simplify $MPV handling
TOOLS/mpv_identify.sh: move script body to a function

Don't prepend each variable with __midentify__, just make them local
to the function.
TOOLS/mpv_identify.sh: various style improvements

Double-quote everything, eliminate unneeded evals and deprecated
`backtits` substitution syntax.
TOOLS/mpv_identify.sh: simplify $MPV handling