Added some server support.

* [404,NEW] New module for displaying a default 404 page.
* [DEBUG,SC] Use of the new SERVER_EXT simplified code a bit.
* [DEV_SERVER,SC] Use of the new SERVER_EXT simplified code a bit.
* [PLDOC_HANDLER,NEW] New module for handling the integration of a plDoc Web server and an application server, both accessible from the same server port.
* [SERVER_EXT,NEW] New module providing some generic code that is used in many servers.