fftools/cmdutils: put stream specifier handling back into cmdutils

Stream specifiers were originally designed exclusively for CLI use and
were not intended to be public API. Handling them in avformat places
major restrictions on how they are used. E.g. if ffmpeg CLI wishes to
override some stream parameters, it has to change the demuxer fields
(since avformat_match_stream_specifier() does not have access to
anything else). However, such fields are supposed to be read-only for
the caller.

Furthermore having this code in avformat restricts extending the
specifier syntax. An example of such an extension will be added in
following commits.

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