Add flags to prevent strdup() on arguments to av_metadata_set2().
I cannot use the same ABI as ffmbc as value 2 is already used in ffmpeg,
besides the name AV_METADATA_NONCONST* makes no sense to me.
Add av_metadata_set2() that takes flags.

Originally committed as revision 20834 to svn://svn.ffmpeg.org/ffmpeg/trunk
