Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

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

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

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

Tiny markup nits.

Add documentation for:
- fenv(3)
- feclearexcept(3), fegetexceptflag(3), feraiseexcept(3),
fesetexceptflag(3), fetestexcept(3)
- fegetround(3), fesetround(3)
- fegetenv(3), feholdexcept(3), fesetenv(3), feupdateenv(3)

Reviewed by: standards@