SYSDB: Wrap sysdb_store_group in a transaction and split it into smaller functions
    
    sysdb_store_group can do several things -- add, rename or update a
    group. It's important they are all done in a single transaction (even
    though the caller should typically start a transaction of his own).
    
    Also split the sysdb_store_group function into two, one that only stores
    a new group and one that changes attributes of an existing group to keep
    the flow easy and avoid two labels in a single function.
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>