Client-side namecheck wildcards.

A client reference identity of ".example.com" matches a server
certificate presented identity that is any sub-domain of "example.com"
(e.g. "www.sub.example.com).

With the X509_CHECK_FLAG_SINGLE_LABEL_SUBDOMAINS flag, it matches
only direct child sub-domains (e.g. "www.sub.example.com").
(cherry picked from commit e52c52f10bb8e34aaf8f28f3e5b56939e8f6b357)