libbb: fix '--help' handling in FEATURE_SH_NOFORK=y

Most BusyBox applets respond to the '--help' option by printing
a usage message.  This is normally handled by busybox_main() so
applet main routines don't have support for '--help'.

In standalone shell mode with FEATURE_SH_NOFORK enabled nofork
applets are invoked directly, bypassing busybox_main().  This
results in inconsistent handling of '--help':

- applets which call getopt() report "unrecognized option '--help'"
  and print help anyway;

- realpath says "--help: No such file or directory" and doesn't
  print help;

- usleep says "invalid number '--help'" and doesn't print help.

Avoid inconsistency by checking for '--help' in run_nofork_applet().

Bug found by Ron Yorston.

function                                             old     new   delta
show_usage_if_dash_dash_help                           -      70     +70
run_nofork_applet                                    347     362     +15
run_applet_no_and_exit                               432     365     -67
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 1/1 up/down: 85/-67)             Total: 18 bytes

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

libbb/vfork_daemon_rexec.c

@@ -109,8 +109,13 @@ int FAST_FUNC run_nofork_applet(int applet_no, char **argv)
 		char *tmp_argv[argc+1];
 		memcpy(tmp_argv, argv, (argc+1) * sizeof(tmp_argv[0]));
 		applet_name = tmp_argv[0];
+
+		/* longjmp's (instead of returning) if --help is seen */
+		show_usage_if_dash_dash_help(applet_no, argv);
+
 		/* 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 {