kern: add devctl_notifyf()

[kevans]

Summary

This version is identical to devctl_notify(), except it fits slightly
less trivial needs by taking a data format string and varargs to go
with it. This lets us avoid extra allocations on top in various
callers, making them a bit less complicated.

[kevans]

Update manpage

[debdrup]

Manual page changes look good to me.

[pauamma_gundo.com]

Other than that, LGTM.

share/man/man9/devctl_notify.9

38

"userland", I think? I don't remember seeing it split.

[kevans]

Rebase past nlsysevent's devctl tap

Alright, hear me out, this isn't the ugliest thing I've ever written. This does
result in a functional difference for nlsysevent, in that we may not tap out an
nlsysevent if we couldn't allocate the sbuf/dei bits for whatever reason. I
don't think it's necessarily a bad difference, but it's a notable one.

[kevans]

Tap in bapt, too, since this could slightly change some nlsysevent semantics in more edgey cases.