Option --cryptoapicert: support issuer name as a selector

Option --cryptoapicert: support issuer name as a selector

- Certificate selection string can now specify a partial
  issuer name string as "--cryptoapicert ISSUER:<string>" where
  <string> is matched as a substring of the issuer (CA) name in
  the certificate.

  Partial case-insensitive matching against the "issuer name" is
  used. Here "issuer name" is a text representation of the RDN's
  separated by commas.

  E.g., "CA, Ontario, Toronto, Acme Inc., IT, Acme Root CA".

  See MSDN docs on CertFindCertificateInStore() with CERT_FIND_ISSUER_STR
  as "FindType" for more details.

  As the order of RDN's is not well-defined[*] and type names like "OU"
  or "CN" are not included, its best to match against a single attribute
  like the CN of the issuer:

  E.g., --cryptoapicert "ISSUER:Acme Root"

[*] Windows appears to order RDN's in the reverse order to which
its written in the certificate but do not rely on this.

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20230128223421.2207802-2-selva.nair@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg26092.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>