libavdevice/decklink: add support for -sources and -sinks arguments

Add support for enumerating the sources/sinks via the ffmpeg
command line options, as opposed to having to create a real pipeline
and use the "-list_devices" option which does exit() after dumping
out the options.

Note that this patch preserves the existing "-list_devices" option,
but now shares common code for the actual enumeration.

Updated to reflect feedback from Marton Balint <cus@passwd.hu>.

Signed-off-by: Devin Heitmueller <dheitmueller@ltnglobal.com>
Signed-off-by: Marton Balint <cus@passwd.hu>