libbb: make '--help' handling more consistent

Running an applet with '--help' as its only argument is treated
as a special case.  If additional arguments follow '--help' the
behaviour is inconsistent:

- applets which call single_argv() print help and do nothing else;

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

- expr says "expr: syntax error" and doesn't print help;

- printenv silently ignores '--help', prints any other variables
  and doesn't print help;

- realpath says "--help: No such file or directory", prints the path
  of any other files and doesn't print help.

If the first argument is '--help' ignore any other arguments and print
help.  This is more consistent and most likely what the user wanted.

See also commit 6bdfbc4cb (libbb: fix '--help' handling in
FEATURE_SH_NOFORK=y).

function                                             old     new   delta
show_usage_if_dash_dash_help                          75      69      -6
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-6)               Total: -6 bytes

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

libbb/appletlib.c

@@ -258,7 +258,6 @@ void lbb_prepare(const char *applet
 	/* Redundant for busybox (run_applet_and_exit covers that case)
 	 * but needed for "individual applet" mode */
 	if (argv[1]
-	 && !argv[2]
 	 && strcmp(argv[1], "--help") == 0
 	 && !is_prefixed_with(applet, "busybox")
 	) {
@@ -940,8 +939,8 @@ void FAST_FUNC show_usage_if_dash_dash_help(int applet_no, char **argv)
 	 && applet_no != APPLET_NO_echo
 #  endif
 	) {
-		if (argv[1] && !argv[2] && strcmp(argv[1], "--help") == 0) {
-			/* Make "foo --help" exit with 0: */
+		if (argv[1] && strcmp(argv[1], "--help") == 0) {
+			/* Make "foo --help [...]" exit with 0: */
 			xfunc_error_retval = 0;
 			bb_show_usage();
 		}