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

40

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