lavu/opt: change the way default pixel and sample format value is set

Use the i64 field rather than the string value. Using a string to set a
default sample/pixel format is weird, also the new interface is more
consistent with the rest of the API.

This is technically an API break, but hopefully there are no applications
using this feature outside of FFmpeg. In order to save backward
compatibility with mixed libraries in case libavutil is updated but not
the other libraries, some ifdeffery hacks are added.

Note that the version check is only performed when class->version != 0,
since if it is not defined then we assume that no version was defined and
the class is not affected by the change.

We will luckily get rid of the hack at the next major bump.
