Prefix all _demuxer, _muxer, _protocol from libavformat and libavdevice.

This also lists the objects from those two libraries as internal (by adding
the ff_ prefix) so that they can then be hidden via linker scripts.
(cherry picked from commit c6610a216e)

libavformat/gopher.c

@@ -119,7 +119,7 @@ static int gopher_read(URLContext *h, uint8_t *buf, int size)
 }
 
 
-URLProtocol gopher_protocol = {
+URLProtocol ff_gopher_protocol = {
     "gopher",
     gopher_open,
     gopher_read,