devctl: allow to register a hook to receive the events

In preparation for netlink sysvent add a function that allow
registering a function to hook the events and also send it via
another kernel module (nlsysvent will be that module).

Prepare a static list of known existing events in the kernel that
will be used to prepopulate nlsysvent multicast group (one per event)

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D37573