make a few struct bb_applet members conditional

rename sllep_and_die -> xfunc_die
make fflush_stdout_and_exit NOFORK-safe
fix some buglets found by randomconfig

coreutils/echo.c

@@ -107,7 +107,9 @@ int bb_echo(char **argv)
 		putchar(' ');
 	}
 
+#if ENABLE_FEATURE_FANCY_ECHO
  newline_ret:
+#endif
 	if (nflag) {
 		putchar('\n');
 	}