samldb: Allow automatic generation of linkIDs and prevent duplicates

samldb: Allow automatic generation of linkIDs and prevent duplicates

As per MS-ADTS 3.1.1.2.3.1, this allows specifying the OID
1.2.840.113556.1.2.50 as the linkID of a new linked attribute in the
schema in order to automatically assign it an unused even linkID.

Specifying the attributeID or ldapDisplayName of an existing forward
link will now also add the new linked attribute as the backlink of that
existing link.

This also prevents adding duplicate linkIDs. Previously, we could run
into issues when trying to delete backlinks with duplicate linkIDs.

Signed-off-by: Bob Campbell <bobcampbell@catalyst.net.nz>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
BUG: https://bugzilla.samba.org/show_bug.cgi?id=11139