> OTOH, we also have sysmon(4), used by envsys(8), powerd(8) and
> wdogctl(8).  [....]

I don't mean this as a troll, but where's the documentation for
sysmon(4)? The fine manpage sysmon(4) doesn't exist, and envsys(4)
says "This API is experimental and may be deprecated at any time".
and "This entire API should be replaced by a sysctl(8) interface
or a kernel events mechanism, should one be developed."

So, er, is envsys/sysmon really suitable as a basis for layering
other subsystems?