Everywhere: Replace a bundle of dbg with dbgln.

These changes are arbitrarily divided into multiple commits to make it
easier to find potentially introduced bugs with git bisect.

Userland/md.cpp

@@ -84,7 +84,7 @@ int main(int argc, char* argv[])
     }
 
     auto buffer = file->read_all();
-    dbg() << "Read size " << buffer.size();
+    dbgln("Read size {}", buffer.size());
 
     auto input = String::copy(buffer);
     auto document = Markdown::Document::parse(input);