always set optarg in getopt_long

the standard getopt does not touch optarg unless processing an option
with an argument. however, programs using the GNU getopt API, which we
attempt to provide in getopt_long, expect optarg to be a null pointer
after processing an option without an argument.

before argument permutation support was added, such programs typically
detected its absence and used their own replacement getopt_long,
masking the discrepency in behavior.

src/misc/getopt_long.c

@@ -51,7 +51,7 @@ static int __getopt_long(int argc, char *const *argv, const char *optstring, con
 
 static int __getopt_long_core(int argc, char *const *argv, const char *optstring, const struct option *longopts, int *idx, int longonly)
 {
-
+	optarg = 0;
 	if (longopts && argv[optind][0] == '-' &&
 		((longonly && argv[optind][1]) ||
 		 (argv[optind][1] == '-' && argv[optind][2])))