proc-cmdline: return proper errors from shall_restore_state()

Let's not eat up errors in shall_restore_state(), but in the consumers
instead, just for the sake of keeping the library calls generic.

src/backlight/backlight.c

@@ -381,7 +381,7 @@ int main(int argc, char *argv[]) {
                 _cleanup_free_ char *value = NULL;
                 const char *clamp;
 
-                if (!shall_restore_state())
+                if (shall_restore_state() == 0)
                         return EXIT_SUCCESS;
 
                 if (!validate_device(udev, device))

src/basic/proc-cmdline.c

@@ -141,5 +141,5 @@ int shall_restore_state(void) {
         if (r == 0)
                 return true;
 
-        return parse_boolean(value) != 0;
+        return parse_boolean(value);
 }

src/rfkill/rfkill.c

@@ -212,7 +212,7 @@ static int load_state(
         assert(udev);
         assert(event);
 
-        if (!shall_restore_state())
+        if (shall_restore_state() == 0)
                 return 0;
 
         r = find_device(udev, event, &device);