Harden raw record processing in DNS plugin
    
    There were cases where DNS plugin was too tolerant in a raw DNS
    record option (--
    DNS record parts options in dnsrecord-mod operations for some
    record without specifying the record that should be updated. It
    also ignored DNS record parts in dnsrecord-add operation when the
    raw DNS record value was already set via --<rrtype>-rec option.
    
    This patch hardens the processing and returns error in both
    described cases to make the processes clearer and more robust.
    
    All these use cases were also covered by new unit tests.
    
    https://fedorahosted.org/freeipa/ticket/2551