- Provide necessary includes, that before came via if.h pollution.
- Remove unnecessary ones.

Sponsored by:	Netflix
Sponsored by:	Nginx, Inc.
New sentence -> new line and some mdoc cleanup.

MFC after:	3 days
Clean up a spurious "." in SEE ALSO.

MFC after:	3 days
Provide necessary includes that before came via if.h.
Provide necessary includes.
[automated] Merge in upstream-github/master