Use non deprecated fastInsert in file.cpp (first of many to come)

Summary:
Avoid the goto using local variables.
Use fastInsert instead of insert.
Fill the ACL of the linked destination.

Test Plan:
The tests pass.
Dolphin works with broken symbolic links.

Reviewers: dfaure, Frameworks

Reviewed By: dfaure

Subscribers: bruns, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D13898