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 a new machine independent varargs.h and use it as a central place
to announce the demise of varargs support in GCC versions 3.3+ and to
direct users to stdarg.h instead.

Fall back to machine/varargs.h for older GCC versions.