MEDIUM: proxy: add mode PR_MODE_PEERS to flag peers frontends

For now we cannot easily distinguish a peers frontend from another one,
which will be problematic to avoid reporting them when stopping their
listeners. Let's add PR_MODE_PEERS for this. It's not supposed to cause
any issue since all non-HTTP proxies are handled similarly now.
diff --git a/src/cfgparse.c b/src/cfgparse.c
index 0bc4b82..12585d0 100644
--- a/src/cfgparse.c
+++ b/src/cfgparse.c
@@ -2355,9 +2355,10 @@
 			cfgerr += proxy_cfg_ensure_no_http(curproxy);
 			break;
 		case PR_MODE_SYSLOG:
+		case PR_MODE_PEERS:
 		case PR_MODES:
 			/* should not happen, bug gcc warn missing switch statement */
-			ha_alert("config : %s '%s' cannot use syslog mode for this proxy.\n",
+			ha_alert("config : %s '%s' cannot use peers or syslog mode for this proxy. NOTE: PLEASE REPORT THIS TO DEVELOPERS AS YOU'RE NOT SUPPOSED TO BE ABLE TO CREATE A CONFIGURATION TRIGGERING THIS!\n",
 				 proxy_type_str(curproxy), curproxy->id);
 			cfgerr++;
 			break;