msghdr.c: print struct mmsghdr as a regular structure

* msghdr.c (decode_mmsghdr): Print names of msg_hdr and msg_len fields.
* tests/mmsg.c (main): Update expected output.
diff --git a/msghdr.c b/msghdr.c
index aa283b0..931d3bd 100644
--- a/msghdr.c
+++ b/msghdr.c
@@ -379,9 +379,9 @@
 	int fetched = fetch_struct_mmsghdr(tcp, addr, &mmsg);
 
 	if (fetched) {
-		tprints("{");
+		tprints("{msg_hdr=");
 		print_msghdr(tcp, &mmsg.msg_hdr, use_msg_len ? mmsg.msg_len : -1UL);
-		tprintf(", %u}", mmsg.msg_len);
+		tprintf(", msg_len=%u}", mmsg.msg_len);
 	} else {
 		printaddr(addr);
 	}