http: Fix decetion of range support in HTTP servers

currently libavformat only allows seeking if a request with "Range:
0-" results in a 206 reply from the HTTP server which includes a
Content-Range header. But according to RFC 2616, the server may also
reply with a normal 200 reply (which is more efficient for a request
for the whole file). In fact Apache HTTPD 2.2.20 has changed the
behaviour in this way and it looks like this change will be kept in
future versions. The fix for libavformat is easy: Also look at the
Accept-Ranges header.