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

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

Guard ptrdiff_t.

Change the definition of NULL on ia64 (for LP64 compilations) from
an int constant to a long constant. This change improves consistency
in the following two ways:
1. The first 8 arguments are always passed in registers on ia64, which
by virtue of the generated code implicitly widens ints to longs and
allows the use of an 32-bit integral type for 64-bit arguments.
Subsequent arguments are passed onto the memory stack, which does
not exhibit the same behaviour and consequently do not allow this.
In practice this means that variadic functions taking pointers
and given NULL (without cast) work as long as the NULL is passed
in one of the first 8 arguments. A SIGSEGV is more likely the
result if such would be done for stack-based arguments. This is
due to the fact that the upper 4 bytes remain undefined.
2. All 64-bit platforms that FreeBSD supports, with the obvious
exception of ia64, allow 32-bit integral types (specifically NULL)
when 64-bit pointers are expected in variadic functions by way of
how the compiler generates code. As such, code that works correctly
(whether rightfully so or not) on any platform other than ia64, may
fail on ia64.

To more easily allow tweaking of the definition of NULL, this commit
removes the 12 definitions in the various headers and puts it in a
new header that can be included whenever NULL is to be made visible.

This commit fixes GNOME, emacs, xemacs and a whole bunch of ports
that I don't particularly care about at this time...

Add the sys/stddef.h header, so that we can have ptrdiff_t
definition in the kernel. It also includes the offsetof()
definition which, according to bde@ should be here and not
in sys/types.h, and the definition of NULL so that we're
closer to make it closer to standard C.

Reviewed by: mike