- Copy stable/10 (r259064) to releng/10.0 as part of the
10.0-RELEASE cycle.
- Update __FreeBSD_version [1]
- Set branch name to -RC1

[1] 10.0-CURRENT __FreeBSD_version value ended at '55', so
start releng/10.0 at '100' so the branch is started with
a value ending in zero.

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.

Scheduled mdoc(7) sweep.

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@