Fix and document kern.signal.coredump devctl

[trasz]

Summary

Rename the kernel.signal.coredump devctl notification
to kern.signal.coredump and document it in devd.conf(5).

At this point we invoke both kernel.signal.coredump
and kern.signal.coredump, for backwards compatibility.

[kib]

You did not provided a single reason for the renaming.

[trasz]

Without renaming we would have both "kern" and "kernel". In devd.conf(5) man page we already have the "kern" top-level, described as "Events related to the kernel"; leaving the old name would require adding a new top level "kernel" - described as "Also events related to the kernel", I guess?

[kib]

In D16323#346585, @trasz wrote:

Without renaming we would have both "kern" and "kernel". In devd.conf(5) man page we already have the "kern" top-level, described as "Events related to the kernel"; leaving the old name would require adding a new top level "kernel" - described as "Also events related to the kernel", I guess?

Yes, the event should be not renamed. It is unfortunate that we have this inconsistency, but I do not see a documentation issue strong enough to warrant the a) the breakage of the interface or b) doubling of the events sent on the core dump.