Subject Common Names can now also be configured to be allowed or
denied, similar to SANs. When a Subject Common Name is not explicitly
allowed or denied, its type will be determined and its value will be
validated according to the constraints for that type of name (i.e. URI).
In Go 1.18 the behavior for looking up domains with non-ASCII
characters was changed to be in accordance with UTS#46
(https://unicode.org/reports/tr46/). There's a slight difference
in how IDNA2003 and IDNA2008 process these. Go 1.18 handles
the deviations in accordance with IDNA2008 now.
This PR improves internationalized domain name handling according
to rules of IDNA and based on the description in RFC 5280, section 7:
https://datatracker.ietf.org/doc/html/rfc5280#section-7.
Support for internationalized URI(s), so-called IRIs, still needs to
be done.
