Please review (2nd) ticket 47823: RFE enforce attributes uniqueness over several subtrees

[thierry bordaz]

Ticket is https://fedorahosted.org/389/ticket/47823

Fix is 
https://fedorahosted.org/389/attachment/ticket/47823/0002-Ticket-47823-attri...
Taking into account Rich review:

* use of (Slapi_DN *) rather that (char *)
  * slapi_ch_free_string rather than slapi_ch_free for (char * strings)

Test case is 
https://fedorahosted.org/389/attachment/ticket/47823/0002-Ticket-47823-test-...