windows plugin directory created and set in cfg.

The windows installer now creates also a directory
for plugins, ususally C:\Program Files\Bareos\Plugins

Also, this directory is set in the plugin directory comment
in the default bareos-fd.conf

Also, we now have a "compatible=no" comment in the fd.conf
that tells the user that for new bareos features this has to
be set to no to disable the Bacula compatible mode.

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