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

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

Remove WARNS=6 lines.

WARNS=6 is already implied at this point.

Fix small documentation mistakes.

Submitted by: brueffer
Approved by: kib (mentor)

Import stdbuf(1) and the shared library it relies on.
This tool changes the default buffering behaviour of standard
stdio streams.

It only works on dynamic binaries. To make it work for static
ones it would require cluttering stdio because there no single
entry point.

PR: 166660
Reviewed by: current@, jhb
Approved by: kib (mentor)
MFC after: 1 week