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.

include/modules.h

@@ -285,6 +285,8 @@ class CoreExport Module : public Extensible
 	 */
 	ModuleVersion GetVersion() const;
 
+	virtual void Prioritize();
+
 	/* Everything below here are events. Modules must ModuleManager::Attach to these events
 	 * before they will be called.
 	 */