crypto/x509: treat hostnames with colons as invalid
Colons are port separators, so it's risky to allow them in hostnames.
Per the CL 231377 rule, if we at least consider them invalid we will not
apply wildcard processing to them, making behavior a little more
predictable.
We were considering hostnames with colons valid (against spec) because
that meant we'd not ignore them in Common Name. (There was at least
one deployment that was putting colons in Common Name and expecting it
to verify.)
Now that Common Name is ignored by default, those clients will break
again, so it's a good time to drop the exception. Hopefully they moved
to SANs, where invalid hostnames are checked 1:1 (ignoring wildcards)
but still work. (If they didn't, this change means they can't use
GODEBUG=x509ignoreCN=0 to opt back in, but again you don't get to use a
legacy deprecated field AND invalid hostnames.)