nofork: fix a bug uncovered by hush testsuite (forgotten fflush)

function                                             old     new   delta
run_nofork_applet                                    280     287      +7

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

libbb/vfork_daemon_rexec.c

@@ -143,6 +143,8 @@ int FAST_FUNC run_nofork_applet(int applet_no, char **argv)
 		applet_name = tmp_argv[0];
 		/* Finally we can call NOFORK applet's main() */
 		rc = applet_main[applet_no](argc, tmp_argv);
+		/* Important for shells: `which CMD` was failing */
+		fflush_all();
 	} else {
 		/* xfunc died in NOFORK applet */
 	}