mxqset: Use initializers with designators

Not all fields of struct argp_option and struct argp are initialized:

mxqset.c:192:5: warning: missing initializer for field ‘group’ of ‘const struct argp_option’ [-Wmissing-field-initializers]
     {"whitelist",               15, "",     0, NULL},
mxqset.c:196:21: warning: missing initializer for field ‘children’ of ‘const struct argp’ [-Wmissing-field-initializers]
 static const struct argp argp = { options, parser, NULL, NULL }

Use structure initializers with designators, which initialize all
remaining fields with their zero values.

mxqset.c

@@ -186,14 +186,14 @@ static error_t parser (int key, char *arg, struct argp_state *state) {
 }
 
 static const struct  argp_option options[] = {
-    {"closed",                  10, NULL,   0, NULL},
-    {"open",                    11, NULL,   0, NULL},
-    {"blacklist",               13, "",     0, NULL},
-    {"whitelist",               15, "",     0, NULL},
+    { .name = "closed",    .key = 10 },
+    { .name = "open",      .key = 11 },
+    { .name = "blacklist", .key = 13 },
+    { .name = "whitelist", .key = 15 },
     {0}
 };
 
-static const struct argp argp = { options, parser, NULL, NULL };
+static const struct argp argp = { .options = options, .parser = parser };
 
 static __attribute__ ((noreturn)) void exit_usage(void) {
     fprintf(stderr,