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.
00ca755231