po/Makefile.in.in: Removing and restoring the config.xml file
broke parallel builds. Build the manpages based on *.xml-config
files instead of *.xml files. The *.xml do not include config.xml
anymore, which permits to run xml2po without needing to remove
config.xml. The config.xml is restored in the *.xml-config files.
* man/groupadd.8.xml: Implementation of the above.
* man/generate_mans.deps: Updated dependencies
