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Closes #11073

In the provided link, SWF params give up to 720P, and VideoConfig
gives 1080P for both best and bestvideo. I guess all Facebook videos
supports HTML5 now, so I remove the old detection for SWF params

Thanks @TwelveCharzz again for studying openload codes

[ci skip]

When doing version probes for ffmpeg, do the
equivalent of calling it as:

    ffmpeg -version <&-

Where <&- is shell syntax for closing stdin before calling the
program. This is roughly equivalent to </dev/null without actually
opening /dev/null.

This prevents ffmpeg -version from hanging when run in the background.
Fixes #955.

The reason is that ffmpeg tries to manipulate stdin to set up terminal
characteristic, and that causes the kernel to suspend the parent
process (youtube-dl).

Note that closing stdin is achieved by calling subprocess.Popen() with
stdin set to subprocess.PIPE and without passing any input to
Popen.communicate(). This is somewhat subtle.