Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

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

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

mdoc(7) police: "The .Fa argument.".

mdoc(7) police: "The .Fn function".

mdoc(7) police: overhaul.

Approved by: re

Add man pages for getcontext()/setcontext(), makecontext()/swapcontext(),
and ucontext_t.

Reviewed by: mini
MFC after: 3 days