BUG/MINOR: add missing modes in proxy_mode_str()

Add the missing PR_MODE_SYSLOG and PR_MODE_PEERS in proxy_mode_str().

Could be backported in every maintained versions.
diff --git a/src/proxy.c b/src/proxy.c
index 946fe13..dbcab47 100644
--- a/src/proxy.c
+++ b/src/proxy.c
@@ -379,6 +379,10 @@
 		return "http";
 	else if (mode == PR_MODE_CLI)
 		return "cli";
+	else if (mode == PR_MODE_SYSLOG)
+		return "syslog";
+	else if (mode == PR_MODE_PEERS)
+		return "peers";
 	else
 		return "unknown";
 }