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/loginutils/su.c b/loginutils/su.c
index e7e0001..de8c18d 100644
--- a/loginutils/su.c
+++ b/loginutils/su.c
@@ -44,7 +44,7 @@
 		But getlogin can fail -- usually due to lack of utmp entry.
 		in this case resort to getpwuid.  */
 		old_user = xstrdup(USE_FEATURE_UTMP(getlogin() ? : ) (pw = getpwuid(cur_uid)) ? pw->pw_name : "");
-		tty = ttyname(2) ? : "none";
+		tty = xmalloc_ttyname(2) ? : "none";
 		openlog(applet_name, 0, LOG_AUTH);
 	}