perform case-insensitive principal search when canonicalization is requested
    
    When canonicalization is requested, the krbprincipalname attribute is searched
    for case-insensitively.
    
    In the case that krbcanonicalname is not set, the matched alias is returned
    with the casing stored in backend, not the one input by client.
    
    Part of https://fedorahosted.org/freeipa/ticket/3864
    
    Reviewed-By: David Kupka <dkupka@redhat.com>
    Reviewed-By: Simo Sorce <ssorce@redhat.com>