devctl: allow to register a hook to receive the events

[imp]

sys/kern/kern_devctl.c

139

why only one?

443

and why here rather than in devctl_queue?

[bapt]

sys/kern/kern_devctl.c

139

because I didn't plan to add more hooks, but I can turn it into a hook.

443

Look at D37574 my goal is to be able to allow consumers to subscribe at the "system" level" so I don't want the message to be preformated.

[bapt]

Remove extra tab

[imp]

sys/kern/kern_devctl.c

443

You're missing all the device events if you do this then. It's too high a level.
And the messages already are pre-formatted by the time they get here, except for the system, subsystem and type that are added.
The data that's passed in is already a bunch of keyword=value pairs that are created using certain rules to avoid quoting issues.

[bapt]

Add all the device events
Add a generic structure for events classification