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

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

MFC r230429:
Add API for obtaining extended machine context states that cannot be
fit into existing mcontext_t.

On i386 and amd64 do return the extended FPU states using
getcontextx(3). For other architectures, getcontextx(3) returns the
same information as getcontext(2).

MFC r230864:
Make the sys/ucontext.h self-contained by changing the return type
of __getcontextx_size(3) from size_t to int.

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

Approved by: re (implicit)

Eliminate double whitespace.

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