MINOR: proxy: make proxy_type_str() recognize peers sections

Now proxy_type_str() will emit "peers section" when the mode is set to
peers, so as to ease sharing more code between peers and proxies.
diff --git a/include/haproxy/proxy.h b/include/haproxy/proxy.h
index 0390772..547c671 100644
--- a/include/haproxy/proxy.h
+++ b/include/haproxy/proxy.h
@@ -92,6 +92,8 @@
  */
 static inline const char *proxy_type_str(struct proxy *proxy)
 {
+	if (proxy->mode == PR_MODE_PEERS)
+		return "peers section";
 	return proxy_cap_str(proxy->cap);
 }