[various] Address a couple of PHP 8.1 notices

- Replace a couple of strstr calls by strpos
- Address part of https://github.com/friendica/friendica/issues/12011#issuecomment-1331012289

dwpost/dwpost.php

@@ -126,7 +126,7 @@ function dwpost_send(App $a, array &$b)
 		return;
 	}
 
-	if (!strstr($b['postopts'],'dwpost')) {
+	if (strpos($b['postopts'] ?? '', 'dwpost') === false) {
 		return;
 	}
 

statusnet/statusnet.php

@@ -366,7 +366,7 @@ function statusnet_hook_fork(App $a, array &$b)
 		}
 	} else {
 		// Comments are never exported when we don't import the GNU Social timeline
-		if (!strstr($post['postopts'], 'statusnet') || ($post['parent'] != $post['id']) || $post['private']) {
+		if (strpos($post['postopts'] ?? '', 'statusnet') === false || ($post['parent'] != $post['id']) || $post['private']) {
 			$b['execute'] = false;
 			return;
 		}