Add an "-xmpphost" option to s_client

 * Many XMPP servers are configured with multiple domains (virtual hosts)
 * In order to establish successfully the TLS connection you have to specify
   which virtual host you are trying to connect.
 * Test this, for example with ::
   * Fail:
       openssl s_client -connect talk.google.com:5222 -starttls xmpp
   * Works:
       openssl s_client -connect talk.google.com:5222 -starttls xmpp -xmpphost gmail.com