In f121d95, the outlink framerate was unconditionally unset.
This breaks/bloats outputs from CFR muxers unless the user explicitly
sets a sane framerate. And the most common invocation for setpts seen in
workflows, our docs and across the web is `PTS-STARTPTS` or others of the
general form `PTS+constant` which preserves the input framerate.
Default value is false, which restores old behaviour.
Fixes#11428
