It has not worked for anything other than fringe codecs (asv1/2, mdec,
mjpeg[b]) since about 2003 and nobody ever noticed or complained. This
sufficiently proves that there are no users of this option who have a
clue of what they are doing, so it is completely useless.

func2_arg is the same as func_arg, except it has one additional
parameter. Change all func_arg callbacks to take that parameter (and
ignore it).

The new name is shorter and more accurate, since this variable is no
longer used only for checking recording time constraint.

This simplifies splitting code for reading from input out of
transcode().