Add initialization routines

This adds a function to register each module. As each module is
require'd, it must call init_register_module() with its name. The module
must also define a function module_init (where module is the module
name).
Improve module handling and support

* Move module registration functions to init/core.
* Overlord help now walks the registered modules to display the short
  help string.
* Command processing checks for a registered module/command and calls
* the corresponding CLI