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

Fix two bugs in the signbit() macro, which was implemented last year:

- It was added to libc instead of libm. Hopefully no programs rely
on this mistake.

- It didn't work properly on large long doubles because its argument
was converted to type double, resulting in undefined behavior.