HEADS UP: coming changes to a devd event

Greetings

I just pushed https://svnweb.freebsd.org/changeset/base/364725 into
-current.

In documenting all the messages that devd can receive from the kernel, I
noticed an inconsistency I'd like to correct. We currently have both
system=kernel and system=kern generated by the kernel. This makes no
sense, imho. I've decided to transition the 'kern' one to 'kernel' because
(a) I think it's less used and (b) I like it less than 'kernel' for this.

The one message is generated by newus on resume. It's redundant since we
also generate one for ACPI. We don't really document either (until
recently), but use the ACPI one in the default devd.conf script. I'm not
changing that at all since it doesn't need adjustment.

This is a heads up that the other one is changing from system=kern to
system=kernel to be more consistent with the other messages generated by
the kernel. I've updated the documentation and added an UPDATING entry.

For a while, we'll generate both system=kern and system=kernel events for
this. Sometime after 13 is branched, I'll remove the system=kern one. If
all goes well, I'll MFC the change to generate both to stable/12 in a few
days, along with the improved docs to devd.conf (which I may do prior to
that), but won't MFC the removal.

I don't think this will cause anybody any grief, and the lead times on
making changes will be at least 6 months for people following -current, and
likely on the scale of years for people following stable branches. Sadly,
it's not simple to add a deprecation notice for current users.

Just thought I'd give a heads up in case this will cause people grief.

Warner