libbb: move nuke_str() from passwd into libbb

function                                             old     new   delta
nuke_str                                               -      15     +15
ask_and_check_password_extended                      215     206      -9
init_main                                            781     771     -10
nuke_str                                              27       -     -27
------------------------------------------------------------------------------
(add/remove: 1/1 grow/shrink: 0/2 up/down: 15/-46)            Total: -31 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/init/init.c b/init/init.c
index 2dcdfd4..c63bba1 100644
--- a/init/init.c
+++ b/init/init.c
@@ -1128,7 +1128,7 @@
 	strncpy(argv[0], "init", strlen(argv[0]));
 	/* Wipe argv[1]-argv[N] so they don't clutter the ps listing */
 	while (*++argv)
-		memset(*argv, 0, strlen(*argv));
+		nuke_str(*argv);
 
 	/* Set up signal handlers */
 	if (!DEBUG_INIT) {