MEDIUM: http: Ignore http-tunnel option on backend

This option is frontends specific, so there is no reason to support it on
backends. So now, it is ignored if it is set on a backend and a warning is
emitted during the startup. The change is quite trivial, but the commit is
tagged as MEDIUM because it is a small breakage with previous versions and
configurations using this options could emit a warning now.
diff --git a/src/cfgparse.c b/src/cfgparse.c
index 2ac22b5..1e73e32 100644
--- a/src/cfgparse.c
+++ b/src/cfgparse.c
@@ -4761,6 +4761,10 @@
 			}
 		}
 		else if (strcmp(args[1], "http-tunnel") == 0) {
+			if (warnifnotcap(curproxy, PR_CAP_FE, file, linenum, args[1], NULL)) {
+				err_code |= ERR_WARN;
+				goto out;
+			}
 			if (alertif_too_many_args_idx(0, 1, file, linenum, args, &err_code))
 				goto out;
 			if (kwm == KWM_STD) {