modulemanager: load(): Return and use the correct module object

util.helpers: After nearly 'fixing' this code, I conclude it instead only deserves a bigger smile

modulemanager: Cleanup some unused variables, imports, whitespace and add a comment.

modulemanager, moduleapi: Replace hooks multitable with an event_handlers map stored in individual modules. Also adds module:hook_object_event() to hook events on any util.events compatible object.

modulemanager: Drop unnecessary prosody_events local

modulemanager: Some reorganisation. Only external change is (should be) that module-unloaded and module-loaded are no longer fired when reloading a module, the new event module-reloaded is fired instead.

modulemanager: Move in-module API functions to core.moduleapi (half the file size, yay)

configmanager: get(): Make section (core) optional (hurrah)

Merge with Markus Kötter

muc - implement per channel history limits - allow configuration via channel settings - store the settings for permanent channels - honor muc max_history_messages from the config as upper limit - only broadcast_message with historic = true if history_length is > 0