Include clientos=Windows in the prelogin request too

Per this comment (#116 (comment)),
`clientos=Windows` (or `=Mac`) is required to get the servers to respond correctly when
using SAML. This is similar to other cases where the official clients and servers
sometimes respond with bizarre misleading errors if `clientos` doesn't have one of the magic
acceptable values (see 7f7d739).

The official clients also include `os-version` (free-form),
`kerberos-support=yes`, and in newer versions `ipv6-support=yes` as well.

auth-globalprotect.c

@@ -446,7 +446,7 @@ static int gpst_login(struct openconnect_info *vpninfo, int portal, struct login
 	for (;;) {
 		/* submit prelogin request to get form */
 		orig_path = vpninfo->urlpath;
-		vpninfo->urlpath = strdup(portal ? "global-protect/prelogin.esp" : "ssl-vpn/prelogin.esp");
+		vpninfo->urlpath = strdup(portal ? "global-protect/prelogin.esp?tmp=tmp&clientVer=4100&clientos=Windows" : "ssl-vpn/prelogin.esp");
 		result = do_https_request(vpninfo, "POST", NULL, NULL, &xml_buf, 0);
 		free(vpninfo->urlpath);
 		vpninfo->urlpath = orig_path;