passwd: rework:

* do not make backup copy by copying (just retain old file)
* correctly fall back to /etc/passwd if user is not in shadow
* fix bug with overlong passwd entries
* be permissive on some kinds of failures
* reduce stack usage
* code size: -500 bytes

libbb/bb_askpass.c

@@ -60,7 +60,7 @@ char *bb_askpass(int timeout, const char * prompt)
 		   (read did not overwrite it) */
 		do {
 			if (passwd[i] == '\r' || passwd[i] == '\n')
-				passwd[i] = 0;
+				passwd[i] = '\0';
 		} while (passwd[i++]);
 	}