libbb: introduce and use xmalloc_ttyname (-32 in bss).
ash: small code shrink

   text    data     bss     dec     hex filename
 793669     504    7524  801697   c3ba1 busybox_old
 793659     504    7492  801655   c3b77 busybox_unstripped

diff --git a/init/mesg.c b/init/mesg.c
index cfb517f..ca230f3 100644
--- a/init/mesg.c
+++ b/init/mesg.c
@@ -25,7 +25,7 @@
 	if (--argc == 0
 	 || (argc == 1 && ((c = **++argv) == 'y' || c == 'n'))
 	) {
-		tty = ttyname(STDERR_FILENO);
+		tty = xmalloc_ttyname(STDERR_FILENO);
 		if (tty == NULL) {
 			tty = "ttyname";
 		} else if (stat(tty, &sb) == 0) {