http: add hack to make streams served by MediaGateway not seekable

These streams are reported as seekable, but all tests show they are not,
and the server merely pretends the streams are seekable. The server
responds with:

    content-range: bytes 0-1999999999/2000000000

Range requests seem to be correctly answered, but the actual data
returned at the same offset is different. Assume this is a bug in the
server software. The server identifies itself as:

    Server: MediaGateway 3.5.2-001

Add a hack that checks the server name, and disables seeking in this
case.

Test URL: http://8283.live.streamtheworld.com:80/CBC_R1_VCR_H_SC

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>