Allow webmail access from custom domain or multiple location

[winux1]

By removing session_path it becomes possible (you can set it up in a custom Nginx config file) to access Roundcube from multiple paths and/or custom domains. I believe it would be a big improvement for many people compared to the security risk that the leaking of session could have.

[winux1]

I think it could close #71 if the user adds something like this to the per domain Nginx conf file:

# Roundcube Webmail configuration.
location / {
         index index.php;
         alias /usr/local/lib/roundcubemail/;
}
location ~ /config/.* {
         # A ~-style location is needed to give this precedence over the next block.
         return 403;
}
location ~ ^/(?!\/)[^/]*\.php$ {
         # note: ~ has precendence over a regular location block
         include fastcgi_params;
         fastcgi_split_path_info ^(/.*)()$;
         fastcgi_index index.php;
         fastcgi_param SCRIPT_FILENAME /usr/local/lib/roundcubemail/$fastcgi_script_name;
         fastcgi_pass php-fpm;

         # Outgoing mail also goes through this endpoint, so increase the maximum
         # file upload limit to match the corresponding Postfix limit.
         client_max_body_size 128M;
}

@ddavness Any thoughts?

Edit: Updated Nginx config for serving webmail on /