avio: add avio_open2, taking an interrupt callback and options

The interrupt callback has to be passed in during opening (setting it
after opening isn't enough), since a blocking open couldn't be
interrupted otherwise.

Options are passed down to procotols and also need to be available
during open() in most cases.

Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavformat/avio_internal.h

@@ -23,6 +23,10 @@
 #include "avio.h"
 #include "url.h"
 
+#include "libavutil/log.h"
+
+extern const AVClass ffio_url_class;
+
 int ffio_init_context(AVIOContext *s,
                   unsigned char *buffer,
                   int buffer_size,