BUG/MINOR: checks: correctly configure the address family and protocol

Currently, mixing an IPv4 and an IPv6 address in checks happens to
work by pure luck because the two protocols use the same functions
at the socket level and both use IPPROTO_TCP. However, they're
definitely wrong as the protocol for the check address is retrieved
from the server's address.

Now the protocol assigned to the connection is the same as the one
the address in use belongs to (eg: the server's address or the
explicit check address).
diff --git a/src/checks.c b/src/checks.c
index fceb2c7..5eb5a76 100644
--- a/src/checks.c
+++ b/src/checks.c
@@ -1509,6 +1509,7 @@
 	struct check *check = t->context;
 	struct server *s = check->server;
 	struct connection *conn = check->conn;
+	struct protocol *proto;
 	int rv;
 	int ret;
 	int expired = tick_is_expired(t->expire, now_ms);
@@ -1573,12 +1574,16 @@
 		/* no client address */
 		clear_addr(&conn->addr.from);
 
-		if (is_addr(&s->check_common.addr))
+		if (is_addr(&s->check_common.addr)) {
 			/* we'll connect to the check addr specified on the server */
 			conn->addr.to = s->check_common.addr;
-		else
+			proto = s->check_common.proto;
+		}
+		else {
 			/* we'll connect to the addr on the server */
 			conn->addr.to = s->addr;
+			proto = s->proto;
+		}
 
 		if (check->port) {
 			set_host_port(&conn->addr.to, check->port);
@@ -1606,8 +1611,8 @@
 		 * connect() when a pure TCP check is used (without PROXY protocol).
 		 */
 		ret = SN_ERR_INTERNAL;
-		if (s->check_common.proto->connect)
-			ret = s->check_common.proto->connect(conn, check->type, (check->type) ? 0 : 2);
+		if (proto->connect)
+			ret = proto->connect(conn, check->type, (check->type) ? 0 : 2);
 		conn->flags |= CO_FL_WAKE_DATA;
 		if (s->check.send_proxy) {
 			conn->send_proxy_ofs = 1;
@@ -2075,15 +2080,16 @@
 			/* no client address */
 			clear_addr(&conn->addr.from);
 
-			if (is_addr(&s->check_common.addr))
+			if (is_addr(&s->check_common.addr)) {
 				/* we'll connect to the check addr specified on the server */
 				conn->addr.to = s->check_common.addr;
-			else
+				proto = s->check_common.proto;
+			}
+			else {
 				/* we'll connect to the addr on the server */
 				conn->addr.to = s->addr;
-
-			/* protocol */
-			proto = protocol_by_family(conn->addr.to.ss_family);
+				proto = s->proto;
+			}
 
 			/* port */
 			if (check->current_step->port)
diff --git a/src/server.c b/src/server.c
index 565a108..51bb85a 100644
--- a/src/server.c
+++ b/src/server.c
@@ -597,6 +597,7 @@
 				}
 
 				newsrv->check_common.addr = *sk;
+				newsrv->check_common.proto = protocol_by_family(sk->ss_family);
 				cur_arg += 2;
 			}
 			else if (!strcmp(args[cur_arg], "port")) {