avutil/opt: add opt_size()

If people want, this could be exported as av_opt_size()
avutil/opt: add av_opt_copy()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/options: use av_opt_copy() in avcodec_copy_context() to copy priv_data

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>