av_set_string() and av_set_string2() do not accept as parameter the
name of a named constant.

This avoids av_set_string2() and av_set_string() to misbehave when
there is an option and a named constant with the same name.

See the thread: "[PATCH] Remove libavcodec/opt.c:set_all_opt()".

Originally committed as revision 16134 to svn://svn.ffmpeg.org/ffmpeg/trunk