Command 'importdnsrecords' (was 'dnsappendrecords') extended to create PTR records.
PTR records will be generated only for type A records.
Only records of type A are checked for valid IP addresses.
Added more descriptive error message for invalid IP addresses.
Merge pull request #635 from xor-xor/t1366

Command 'importdnsrecords' (was 'dnsappendrecords') extended to create PTR records.