LibCore: Create ConfigFiles with an already-open File

This moves the fallible action of opening the file, from the
constructor, into the factory methods which can propagate any errors.

The wrinkle here is that failure to open a ConfigFile in read-only mode
is allowed (and expected, since the file may not exist), and treated as
if an empty file was successfully opened.

Userland/Libraries/LibCore/ConfigFile.h

@@ -60,8 +60,7 @@ public:
     String filename() const { return m_file->filename(); }
 
 private:
-    explicit ConfigFile(String const& filename, AllowWriting);
-    explicit ConfigFile(String const& filename, int fd);
+    ConfigFile(String const& filename, NonnullRefPtr<File> open_file);
 
     void reparse();