Preserve user principal aliases during rename operation
    
    When a MODRDN is performed on the user entry, the MODRDN plugin resets both
    krbPrincipalName and krbCanonicalName to the value constructed from uid. In
    doing so, hovewer, any principal aliases added to the krbPrincipalName are
    wiped clean. In this patch old aliases are fetched before the MODRDN operation
    takes place and inserted back after it is performed.
    
    This also preserves previous user logins which can be used further for
    authentication as aliases.
    
    https://fedorahosted.org/freeipa/ticket/6028
    
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Simo Sorce <ssorce@redhat.com>