Add helper function to check for krb5 oid
    
    The krb5 mechanism supports multiple oids for historical reasons.
    Add a function to generically check if a mech oid is any of the krb5
    mechanism known oids for functions that do not care which exact oid is
    being used of the krb5 family.
    
    Signed-off-by: Simo Sorce <simo@redhat.com>
    Reviewed-by: Günther Deschner <gdeschner@redhat.com>