crypto/x509: ignore Common Name when it does not parse as a hostname

crypto/x509: ignore Common Name when it does not parse as a hostname

The Common Name is used as a hostname when there are no Subject
Alternative Names, but it is not restricted by name constraints. To
protect against a name constraints bypass, we used to require SANs for
constrained chains. See the NameConstraintsWithoutSANs error.

This change ignores the CN when it does not look like a hostname, so we
can avoid returning NameConstraintsWithoutSANs.

This makes it possible to validate certificates with non-hostname CN
against chains that use name constraints to disallow all names, like the
Estonian IDs.

Updates #24151

Change-Id: I798d797990720a01ad9b5a13336756cc472ebf44
Reviewed-on: https://go-review.googlesource.com/123355
Reviewed-by: Adam Langley <agl@golang.org>