MINOR: server: Forbid server definitions in frontend sections

An fatal error is now reported if a server is defined in a frontend
section. til now, a warning was just emitted and the server was ignored. The
warning was added in the 1.3.4 when the frontend/backend keywords were
introduced to allow a smooth transition and to not break existing
configs. It is old enough now to emit an fatal error in this case.

This patch is related to the issue #1043. It may be backported at least as
far as 2.2, and possibly to older versions. It relies on the previous commit
("MINOR: config: Add failifnotcap() to emit an alert on proxy capabilities").

(cherry picked from commit e3bdc81f8aeb02b5c92b5d10fc9352daaea1cf20)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
diff --git a/src/server.c b/src/server.c
index 79de104..3d234f1 100644
--- a/src/server.c
+++ b/src/server.c
@@ -1923,8 +1923,10 @@
 			err_code |= ERR_ALERT | ERR_FATAL;
 			goto out;
 		}
-		else if (warnifnotcap(curproxy, PR_CAP_BE, file, linenum, args[0], NULL))
-			err_code |= ERR_WARN;
+		else if (failifnotcap(curproxy, PR_CAP_BE, file, linenum, args[0], NULL)) {
+			err_code |= ERR_ALERT | ERR_FATAL;
+			goto out;
+		}
 
 		/* There is no mandatory first arguments for default server. */
 		if (srv && parse_addr) {