windows config files admin-only access

Windows configuration files were accessible by
normal users until now.

Now we use the NSIS AccessControl Plugin
(http://nsis.sourceforge.net/AccessControl_plug-in)
to limit the access to the Administrator only.

This also means that if you want to edit the
config file, this has to happen as administrator.

Fixes #208: Look into possibility to limit access rights on %APPDATA%/Bareos to administrators only.

Signed-off-by: Marco van Wieringen <marco.van.wieringen@bareos.com>