implement subject alternative names support for SSL connections

I'd like to propose a patch for checking subject alternative names entry in the SSL certificate for DNS names during SSL authentication. 

When the client PGSSLMODE is set to verify-full, the common name (CN) of the PostgreSQL server in the certificate  is matched against the actual host name supplied by the client. A successful connection is only established when those names match.

If a failover schema with a floating IP is used, a single DNS name may point to different hosts after failover. In order to maintain the correct pair of (client connection FQDN, FQDN in the certificate) the certificate from the master should also be transferred to the slave. Unfortunately, it makes failover more complicated, since the server restart is required when the new certificate is installed.

The other option is to issue a single certificate covering all the hosts that may participate in the failover.

So far the only way to cover more than one server name with a single certificate is to use wildcards in the server common name, i.e. *.db.example com. Unfortunately, this schema only works for names like master.db.example.com, slave.db.example.com, but not for the example.com and db-master.example.com and db-slave.example.com or other more complex naming schemas.

The other way to issue a single certificate for multiple names is to set the subject alternative names, described in the RFC 3280 4.2.1.7. SAN allows binding multiple identities to the certificate, including DNS names. For the names above the SAN with look like this:

X509v3 Subject Alternative Name: 

         DNS:db-master.example.com, DNS:db-slave.example.com.

At the moment PostgreSQL doesn't support SANs, but should, according to the following comment in the code  in verify_peer_name_matches_certificate:

/*

 * Extract the common name from the certificate.

The attached patch adds support for checking the client supplied names against the dNSName-s in SAN, making it easier to set secure HA environments using PostgreSQL. If SAN section is present, the DNS names from that section are checked against the peer name in addition to checking the common name (CN) from the certificate. The match is successful if at least one of those names match the name supplied by the peer.

I gave it a spin and it works in our environment (Linux database servers, Linux and Mac clients). I don't have either Windows or old versions of OpenSSL, it's not tested against those systems.

I'd appreciate your feedback.

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