[fikfap] support main page post URLs (#9026)

* Update fikfap.py to allow for extracting a single post from the main page
    Current post extractor only works on links to posts
    on user pages but not on direct links to posts
* include 'singlepost' logic into existing 'post' extractor

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Co-authored-by: Mike Fährmann <mike_faehrmann@web.de>

gallery_dl/extractor/fikfap.py

@@ -67,20 +67,17 @@ class FikfapExtractor(Extractor):
 
 class FikfapPostExtractor(FikfapExtractor):
     subcategory = "post"
-    pattern = BASE_PATTERN + r"/user/(\w+)/post/(\d+)"
+    pattern = BASE_PATTERN + r"/(?:user/\w+/)?post/(\d+)"
     example = "https://fikfap.com/user/USER/post/12345"
 
     def posts(self):
-        user, pid = self.groups
+        pid = self.groups[0]
 
-        url = f"{self.root_api}/profile/username/{user}/posts"
-        params = {"amount" : "1", "startId": pid}
-        posts = self.request_api(url, params)
+        url = f"{self.root_api}/posts/{pid}"
+        post = self.request_api(url, None)
 
-        pid = int(pid)
-        for post in posts:
-            if post["postId"] == pid:
-                return (post,)
+        if post["postId"] == int(pid):
+            return (post,)
         raise exception.NotFoundError("post")