Also update BUILD.txt

This task allows us to rebuild memberuid and memberof attributes throughout the
database. This way we can upgrade from version 0.4 databases that didn't
generate and store memberuid.
The task can be invoked by adding a speaicl named entry to the ldb file.
The entry dn to use is: @MEMBEROF-REBUILD, the entry has no attributes and any
attribute is ignored at present.
The entry will not be stored in the database but will just trigger the task to
execute a rebuild of the memberof and memberuid attributes

This patch adds a new generated attribute to every group that has
direct or indirect members.
This attribute is called memberuid and contains the name of the users
that are directo or indirect members of this group.
This is done to greatly speed up group enumerations when NSS reads groups
off the cache.

A loop was badly built and was skipping entries.
This left some memberof attributes in place that should have been removed.

Allow to try to set members that do not actually exist.
In that case simply remove them when we find out they are not real
entries.

Fix ldb debug to avoid printing the debug function name, it's useless.

This is part of a set of patches to rewrite sysdb to a hopefully better
API, that will also let use use tevent_req async style calls to manipulate
our cache.