gpo: Add a --generalize to the backup command

gpo: Add a --generalize to the backup command

This normally prints out the entities in DTD form to be given to the restore
command with --entities. Specifying --entities during the backup conveniently
writes these entities to a file. Generalizing occurs after the standard backup
on the XML files, which will then re-write the XML file.

There are a number of files which can be further handled, including many of the
preferences XML files. This will require more annotation and parsing.

Signed-off-by: Garming Sam <garming@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>