[SSSD] [PATCH] Ghost user related fixes to the memberof plugin

[Jakub Hrozek]

Hi,

this thread obsoletes the previous memberof related threads into a single one
to make it easier for the reviewers to apply the patches without having to
guess the dependencies. The patches should apply cleanly on origin/master. In
addition to bringing all the patches together, this set also addresses some
of Simo's concerns, in particular the ghost users are better explained in
the big comment blocks above each major memberof operation.

The patches implement the missing operations of the memberof plugin
related to ghost users - delete and modify. The mod operation is given
some special care because not only we need to propagate the modification
to parent groups, but we also need to retain the inherited ghost attributes.

The intent is to fix issues such as #1652. Most patches also include
unit tests to make sure we don't break existing memberof plugin
functionality.

More information is in the patches themselves or in the commit messages.