Add data field for A6 record
    
    Since A6 is an obsolete RR type, no DNS part option was created.
    This is, however, not consistent with the rest of per-type API
    and may cause problems. This patch adds at least a DNS part for
    raw A6 record data so that the record type is treated consistently.
    
    This patch also fixes interactive mode for A6 records. Their data
    were not detected correctly as dnsrecord_add didn't expect
    a number in DNS part option name.
    
    https://fedorahosted.org/freeipa/ticket/2309