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CAs like Let's Encrypt do not put O and OU into the Subject's DN.
Similarly, O and OU are often used to indicate Domain Validated or
Organization Validation as opposed to the actual OU.

Issuer CN often contains the issuer's server or as an indication of
Extended Validation certificate as opposed to the actual issuer
organization.

The Hostname part as extracted from the Subject is also confusing, as
in the case of a hostname mismatch, the Subject's CN, which
`nsm-format-certificate' naively calls the Hostname, will not actually
match the hostname in the problem preamble.

* lisp/net/nsm.el (nsm-format-certificate): Show full DN of Issuer and
  Subject.  Remove Hostname.
  (nsm-certificate-part, nsm-parse-subject): Removed.