Refuse to redirect to a non-https URL

Refuse to redirect to a non-https URL

Not good:
$ ./openconnect www.cam.ac.uk
Attempting to connect to 131.111.8.46:443
SSL negotiation with www.cam.ac.uk
Connected to HTTPS on www.cam.ac.uk
GET https://www.cam.ac.uk/
Got HTTP response: HTTP/1.1 301 Moved Permanently
GET https://www.cam.ac.uk/http://www.cam.ac.uk:80/
Got HTTP response: HTTP/1.1 301 Moved Permanently
GET https://www.cam.ac.uk/http://www.cam.ac.uk:80/http://www.cam.ac.uk:80/
Got HTTP response: HTTP/1.1 301 Moved Permanently
GET https://www.cam.ac.uk/http://www.cam.ac.uk:80/http://www.cam.ac.uk:80/http://www.cam.ac.uk:80/

OK, I asked it to do a stupid thing, but a polite refusal is much better.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>