SYSDB: Handle user and group renames better
Fixes a regression in the local domain tools where sss_groupadd no longer
detected a GID duplicate. The check for EEXIST is moved one level up into
more high level function.
The patch also adds the same rename support for users. I found it odd that
we allowed a rename of groups but not users. There is a catch when storing
a user -- his cached password would be gone. I think that renaming a user
is such a rare operation that it's not severe, plus there is a warning in