Added an entry point to the modules init stuff.

Added an entry point to the modules init stuff.
The new entry point is defined by the macro
CTDL_MODULE_UPGRADE(module_name).
This entry point works in much the same way as the init entry point
except it is called before the init entry point. This entry point is
intended for use as its name implies. The module should do any upgrade
stuff it needs to do before it can initialise.
serv_upgrade.c demonstrates the useage.