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/cfgparse.c b/src/cfgparse.c
index 10d8f55..cfacb1e 100644
--- a/src/cfgparse.c
+++ b/src/cfgparse.c
@@ -3221,7 +3221,7 @@
 			}
 
 			/* this will also properly set the transport layer for prod and checks */
-			if (newsrv->use_ssl || newsrv->check.use_ssl) {
+			if (newsrv->use_ssl == 1 || newsrv->check.use_ssl == 1 || (newsrv->proxy->options & PR_O_TCPCHK_SSL)) {
 				if (xprt_get(XPRT_SSL) && xprt_get(XPRT_SSL)->prepare_srv)
 					cfgerr += xprt_get(XPRT_SSL)->prepare_srv(newsrv);
 			}
@@ -4009,7 +4009,7 @@
 				p = curpeers->remote;
 				while (p) {
 					if (p->srv) {
-						if (p->srv->use_ssl && xprt_get(XPRT_SSL) && xprt_get(XPRT_SSL)->prepare_srv)
+						if (p->srv->use_ssl == 1 && xprt_get(XPRT_SSL) && xprt_get(XPRT_SSL)->prepare_srv)
 							cfgerr += xprt_get(XPRT_SSL)->prepare_srv(p->srv);
 					}
 					p = p->next;