Fix plugin install into right directory.
Allow upgrade on windows

Now we check if bareos is already installed on the windows machine.
If so, we call the uninstaller of the previous version and install
ourselves afterwards.

The newer uninstaller keep the configuration files as the uninstaller is
called accordingly.

The dialogs for parametrizing the configuration and the dialog
for the config snipped are not shown, as the existing configuration
files will be used.

Still the user has to choose the same components during the upgrade
as were installed before.

fixes #225: Windows installer can't upgrade in place
Fix windows installer.

Explicitly do SetOutPath to the right directory for all sections
so files end up in the right directory and not in the last set output
dir. Also autoselect the install of the plugins. And some cleanup of
old debug and unneeded comments.
Fix some complains by NSIS.

And document why we have two libgcc names (32/64 bits).