validate.go: Remove "_" warning if CNAME is a _domainkey.
validate.go: Remove "_" warning if CNAME is a _domainkey.

  * "_" is permitted in label, if we suspect the target is a TXT.
  * Improve warning text if "." is required at end of target.
  * Move common factor assert_no_enddot(label) outside of case statement.

mathieuherbert/dnscontrol,mathieuherbert/dnscontrol,rbelnap/dnscontrol,whs/dnscontrol,StackExchange/dnscontrol,whs/dnscontrol,StackExchange/dnscontrol,rbelnap/dnscontrol,StackExchange/dnscontrol