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

Add <stdalign.h> and <stdnoreturn.h>.

Even though these header files make little sense to me, they are part of
the standard. By including these header files, you can simply use
`alignas', `alignof' and `noreturn' instead of the underscore-prefixed
versions.