Increase size of 'cname' member of struct resourcerecord to 1024 from 64

This was causing DKIM entries (http://tools.ietf.org/html/rfc5585#page-12) to be truncated, since they are frequently longer than 64 chars.

DKIM signatures are the same as those in RFC 5322 (internet message format), which in the text says they may be as large as 998 chars (http://tools.ietf.org/html/rfc5322#section-2.1.1).

1024 seems like a reasonable, round number.