Keep track of loading/unloading modules

[barometz]

Currently there's no callback or even a message defined for when a module is loaded or unloaded. This will be necessary to facilitate runtime loading and unloading, for instance for "XXX has loaded" or "YYY unexpectedly died".

Modules should have messages available to announce their unloading (so they can be removed from the module table) their loading (to tell other modules or a user that something's come online). The core should also notice when an executable module unexpectedly loses its transport.

[barometz]

core now has the node_loaded and node_unloaded hooks available. Together with the type:nodes request nodes can get all the info they need in this regard. Unexpected transport deaths aren't handled yet, though.