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

doc/apps/s_client.pod

@@ -37,6 +37,7 @@ B<openssl> B<s_client>
 [B<-bugs>]
 [B<-cipher cipherlist>]
 [B<-starttls protocol>]
+[B<-xmpphost hostname>]
 [B<-engine id>]
 [B<-tlsextdebug>]
 [B<-no_ticket>]
@@ -225,6 +226,13 @@ send the protocol-specific message(s) to switch to TLS for communication.
 B<protocol> is a keyword for the intended protocol.  Currently, the only
 supported keywords are "smtp", "pop3", "imap", "ftp" and "xmpp".
 
+=item B<-xmpphost hostname>
+
+This option, when used with "-starttls xmpp", specifies the host for the
+"to" attribute of the stream element.
+If this option is not specified, then the host specified with "-connect"
+will be used.
+
 =item B<-tlsextdebug>
 
 print out a hex dump of any TLS extensions received from the server.