Disallow names that start with '.' in IsDomainName()

[shane-kerr]

We noticed that IsDomainName() was allowing names that start with at .. This is an error, which this PR fixes.

[tmthrgd]

// XXX: The logic in this function was copied from packDomainName and
// should be kept in sync with that function.

^See the comment at the start of the function.

[shane-kerr]

// XXX: The logic in this function was copied from packDomainName and
// should be kept in sync with that function.

^See the comment at the start of the function.

Ah, okay. I've updated packDomainName(). I didn't see any relevant tests in msg_test.go and I also didn't add any. Hopefully that is okay.

[miekg]

thanks both, comment is acted upon. merging