If the user has certificates in $HOME/.postgresql/postgresql.crt
and $HOME/.postgresql/postgresql.key exist, they are provided
to the server.  The certificate used to sign this cert must be
known to the server, in $DataDir/root.crt.  If successful, the
cert's "common name" is logged.

Client certs are not used for authentication, but they could be
via the port->peer (X509 *), port->peer_dn (char *) or
port->peer_cn (char *) fields.  Or any other function could be
used, e.g., many sites like the issuer + serial number hash.

Bear Giles