Let OverwritePolicy take care of asking users whether to overwrite a file
Refactor the interaction between OverwritePolicy and Output by moving
obtainOverwritePermission from FileOutput to OverwritePolicy. This
allows us to remove the ugly hack to check the use count of a shared
pointer. And it paves the way for asking users for overwrite permission
independent of FileOutput.
- GnuPG-bug-id: T5478