avio: Fix the deprecated fallback URL-prefixed open flags

avio: Fix the deprecated fallback URL-prefixed open flags

While deprecated, they're totally useless as long as their values
are different from the AVIO_FLAG values that are used internally.
Currently, this leads to old libav applications still compiling
correctly (since we haven't removed the fallback wrappers), but
failing since the functions internally compare to the new AVIO_FLAG
values.

These should be removed at some point, but they aren't removed yet.
The intent is to be able to recompile an old application against
the new ABI without modifying the code, and this doesn't work
currently.

Signed-off-by: Martin Storsjö <martin@martin.st>