if the client enabled --ssl-verify-server-cert, then
the server certificate is verified as follows:

* if --ssl-ca or --ssl-capath were specified, the cert must have
  a proper signature by the specified CA (or CA in the path)
  and the cert's hostname must match the server's hostname.
  If the cert isn't signed or a hostname is wrong - the
  connection is aborted.

* if MARIADB_OPT_TLS_PEER_FP was used and the fingerprint matches,
  the connection is allowed, if it doesn't match - aborted.

* If the connection uses unix socket or named pipes - it's allowed.
  (consistent with server's --require-secure-transport behavior)

otherwise the cert is still in doubt, we don't know if we can trust
it or there's an active MitM in progress.

* If the user has provided no password or the server requested an
  authentication plugin that sends the password in cleartext -
  the connection is aborted.

* Perform the authentication. If the server accepts the password,
  it'll send SHA2(scramble || password hash || cert fingerprint)
  with the OK packet.

* Verify the SHA2 digest, if it matches - the connection is allowed,
  otherwise it's aborted.