gfile: Limit access to files when copying

file_copy_fallback creates new files with default permissions and
set the correct permissions after the operation is finished. This
might cause that the files can be accessible by more users during
the operation than expected. Use G_FILE_CREATE_PRIVATE for the new
files to limit access to those files.
10 jobs for wip/oholy/copy-permissions in 11 minutes and 48 seconds (queued for 1 second)