avio: remove AVIO_* access symbols in favor of new AVIO_FLAG_* symbols

Make AVIO_FLAG_ access constants work as flags, and in particular fix
the behavior of functions (such as avio_check()) which expect them to
be flags rather than modes.

This breaks API.

libavformat/output-example.c

@@ -492,7 +492,7 @@ int main(int argc, char **argv)
 
     /* open the output file, if needed */
     if (!(fmt->flags & AVFMT_NOFILE)) {
-        if (avio_open(&oc->pb, filename, AVIO_WRONLY) < 0) {
+        if (avio_open(&oc->pb, filename, AVIO_FLAG_WRITE) < 0) {
             fprintf(stderr, "Could not open '%s'\n", filename);
             exit(1);
         }