Move peer_cert handling to openconnect_open_https()

Move peer_cert handling to openconnect_open_https()

There's no real need to do this in openconnect_obtain_cookie(). It doesn't
really matter if we do it for other connections, since any connections we
make *after* obtaining the cookie will be to the same server anyway.

This moves another OpenSSL-specific snippet out of what should be generic
code.

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