avformat/hls: enable http_multiple only for http/1.1 servers

Some http/1.0 implementations, like python's SimpleHTTPServer, can only support one client connection at a time. Making a second request while the first is still connected leads to a deadlock.

This change enables multiple connections for http/1.1 servers only, which need to support keepalive by default and should have no problem with concurrent requests.

Signed-off-by: Aman Gupta <aman@tmm1.net>