Print struct iovec as a regular structure

* io.c (print_iovec): Print names of iov_base and iov_len fields.
* tests/aio.c (main): Update expected output.
* tests/inet-cmsg.c (main): Likewise.
* tests/mmsg.c (main): Likewise.
* tests/msg_name.c (test_msg_name): Likewise.
* tests/preadv-pwritev.c (main): Likewise.
* tests/preadv2-pwritev2.c (main): Likewise.
* tests/preadv.c (main): Likewise.
* tests/pwritev.c (main): Likewise.
* tests/readv.c (main): Likewise.
* tests/recvmsg.c (main): Likewise.
* tests/vmsplice.c (main): Likewise.
* tests/scm_rights-fd.test: Likewise.
* tests/uio.expected: Likewise.
diff --git a/io.c b/io.c
index 7ddeb78..58323a7 100644
--- a/io.c
+++ b/io.c
@@ -77,7 +77,7 @@
 		iov = elem_buf;
 	}
 
-	tprints("{");
+	tprints("{iov_base=");
 
 	len = iov[1];
 
@@ -99,7 +99,7 @@
 			break;
 	}
 
-	tprintf(", %lu}", iov[1]);
+	tprintf(", iov_len=%lu}", iov[1]);
 
 	return true;
 }