Fix permissions when copying files

This reverts commit 7863f595, I was wrong :)

Reading the code some more (in copyjob, file_copy and file_unix), the
permissions arg is '-1' when we shouldn't touch the dest permissions at all,
i.e. leave them as the system default permissions for newly created files
(umask ...etc).

The scenarios are:
- calling KIO::copy() A to B, permissions are preserved by default, i.e. the
  job copies A's permissions to the permissions arg when it calls file_copy
- calling KIO::copy() A to B, and calling setDefaultPermissions(true),
  the job sets the permissions arg to '-1' when it calls file_copy
- calling KIO::file_copy() directly, the you have full control, if you want
  to preserve the permissions use A's permissions as the permissions arg,
  otherwise use '-1' to use the system default permissions

ACL attributes are handled implicitly at the moment, which isn't ideal but
can be improved later on.

BUG: 404777
CCBUG: 103331

FIXED-IN: 5.87