Improve keytab code to select the right principal.
    
    Whe requesting a keytab the salt used is the NORMAL type (for backwards and AD
    compatibility), however since we added alias support we need to search for the
    krbCanonicalName in preference, hen nothing is specified, and for the requested
    principal name when a getkeytab operation is performed. This is so that the
    correct salt can be applied. (Windows AD uses some peculiar aliases for some
    special accounts to generate the salt).
    
    Signed-off-by: Simo Sorce <simo@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>