Move nick register event to make it easier for modules to determine if a newly registered nick is confirmed or not. Add confirm event.
Fix module event prioritization which was broken with the newer
auto-attach event stuff.

Also remove logically dead code from os_defcon, and make os_defcon akill
similar masks to os_session when enforcing session akills.

Fixes #1618 which relies on os_session to be prioritized before
os_defcon.