Limit scope of CN name constraints

Don't apply DNS name constraints to the subject CN when there's a
least one DNS-ID subjectAlternativeName.

Don't apply DNS name constraints to subject CN's that are sufficiently
unlike DNS names.  Checked name must have at least two labels, with
all labels non-empty, no trailing '.' and all hyphens must be
internal in each label.  In addition to the usual LDH characters,
we also allow "_", since some sites use these for hostnames despite
all the standards.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tim Hudson <tjh@openssl.org>