Clarify usage of aio(4) with kqueue(2)

Reviewed by: jhb
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D11299

MFC 302899: Add documentation for the sigevent structure.

- Add a sigevent(3) manpage to give a general overview of the sigevent
structure and the available notification mechanisms.
- Document that AIO requests contain a nested sigevent structure that can
be used to request completion notification.
- Expand the sigevent details in other manuals to note details such as
the extra values stored in a queued signal's information or in a posted
kevent.

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Re-encode files from ISO-8859-1 to UTF-8

Cross reference lio_listio(2).

mdoc(7) police: nit.

Approved by: re

* Modernize aio(4), providing instructions for static and dynamic kernel
linking.

* Fix disorder in the SEE ALSO sections of aio_*(2).

* Remove unnecessary cross-references from the SEE ALSO sections of
aio_*(2); config(8), kldload(8) and kldunload(8) are cross-referenced
from aio(4).

* Remove the KERNEL OPTIONS sections from aio_*(2), now that these
pages cross-reference aio(4), which contains suitable kernel linking
reference material.

Add a simple man page for the aio facility.