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/include/libbb.h b/include/libbb.h
index e6767e3..e0541a7 100644
--- a/include/libbb.h
+++ b/include/libbb.h
@@ -1144,6 +1144,7 @@
 extern void print_login_issue(const char *issue_file, const char *tty) FAST_FUNC;
 extern void print_login_prompt(void) FAST_FUNC;
 
+char *xmalloc_ttyname(int fd) FAST_FUNC;
 /* NB: typically you want to pass fd 0, not 1. Think 'applet | grep something' */
 int get_terminal_width_height(int fd, unsigned *width, unsigned *height) FAST_FUNC;