Add a trigger to (un)register system user/groups.

The following vars can be used for this:

- system_accounts="foo blah"
- foo_homedir, foo_shell, foo_descr, foo_groups.

--HG--
extra : convert_revision : bc7d002e00abc5c84f83a3716a8ecf97f9c9ff24

TODO

@@ -3,4 +3,3 @@
 - Use binary pkgs for required build dependencies.
 - Make src packages self contained, without requiring to download 
   the whole repository.
-- Add a trigger to (un)register system user/groups.