BUG/MINOR: checks: Respect the no-check-ssl option

This options is used to force a non-SSL connection to check a SSL server or to
invert a check-ssl option inherited from the default section. The use_ssl field
in the check structure is used to know if a SSL connection must be used
(use_ssl=1) or not (use_ssl=0). The server configuration is used by default.

The problem is that we cannot distinguish the default case (no specific SSL
check option) and the case of an explicit non-SSL check. In both, use_ssl is set
to 0. So the server configuration is always used. For a SSL server, when
no-check-ssl option is set, the check is still performed using a SSL
configuration.

To fix the bug, instead of a boolean value (0=TCP, 1=SSL), we use a ternary value :

  * 0  = use server config
  * 1  = force SSL
  * -1 = force non-SSL

The same is done for the server parameter. It is not really necessary for
now. But it is a good way to know is the server no-ssl option is set.

In addition, the PR_O_TCPCHK_SSL proxy option is no longer used to set use_ssl
to 1 for a check. Instead the flag is directly tested to prepare or destroy the
server SSL context.

This patch should be backported as far as 1.8.

(cherry picked from commit f61f33a1b274c2a42afd96aab19ee8e1d8b121cc)
[wt: minor context adjustments]
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 8a5bf35ce7b690353a6adef55cebb4c07d76bf02)
Signed-off-by: Willy Tarreau <w@1wt.eu>
diff --git a/src/checks.c b/src/checks.c
index 13b02bf..2eaac4d 100644
--- a/src/checks.c
+++ b/src/checks.c
@@ -1393,7 +1393,7 @@
 	default:
 		/* good connection is enough for pure TCP check */
 		if ((conn->flags & CO_FL_CONNECTED) && !check->type) {
-			if (check->use_ssl)
+			if (check->use_ssl == 1)
 				set_server_check_status(check, HCHK_STATUS_L6OK, NULL);
 			else
 				set_server_check_status(check, HCHK_STATUS_L4OK, NULL);
@@ -2382,7 +2382,7 @@
 		if (check->result == CHK_RES_UNKNOWN) {
 			/* good connection is enough for pure TCP check */
 			if ((conn->flags & CO_FL_CONNECTED) && !check->type) {
-				if (check->use_ssl)
+				if (check->use_ssl == 1)
 					set_server_check_status(check, HCHK_STATUS_L6OK, NULL);
 				else
 					set_server_check_status(check, HCHK_STATUS_L4OK, NULL);
@@ -3686,7 +3686,8 @@
 	 * default, unless one is specified.
 	 */
 	if (!chk->port && !is_addr(&chk->addr)) {
-		chk->use_ssl |= (srv->use_ssl || (srv->proxy->options & PR_O_TCPCHK_SSL));
+		if (!chk->use_ssl)
+			chk->use_ssl = srv->use_ssl;
 		chk->send_proxy |= (srv->pp_opts);
 	}