Fixed incompatibility of x509 auth with nginx

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

This commit fixes x509 authentication when using nginx as web server.
X509AuthenticationListener depends on the SSL_CLIENT_S_DN_Email field being set.
As is, this field is specific to Apache and cannot be provided by nginx. nginx
can provide the DN of the certificate in SSL_CLIENT_S_DN. If the email field is
not set, the listener tries to extract the email address from the DN.