callout(9): adopt old references to timeout(9)

timeout(9) was removed a couple of years ago; all consumers now use the
callout(9) interface.

Explicitly do not bump .Dd anywhere, as this is not a content or
semantic change.

Reviewed by: markj, jhb, Pau Amma <pauamma@gundo.com>
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39136