lavf: reorganize URLProtocols

Instead of a linked list constructed at av_register_all(), store them
in a constant array of pointers.

Since no registration is necessary now, this removes some global state
from lavf. This will also allow the urlprotocol layer caller to limit
the available protocols in a simple and flexible way in the following
commits.

libavformat/unix.c

@@ -141,7 +141,7 @@ static int unix_get_file_handle(URLContext *h)
     return s->fd;
 }
 
-URLProtocol ff_unix_protocol = {
+const URLProtocol ff_unix_protocol = {
     .name                = "unix",
     .url_open            = unix_open,
     .url_read            = unix_read,