Porting fixes for merged specpdl and backtrace stacks.

Porting fixes for merged specpdl and backtrace stacks.

In particular this ports to 32-bit sparc Sun cc.
* eval.c (init_eval_once, grow_specpdl): Allocate a specbinding
array with a dummy element at specpdl[-1], so that its address can
be taken portably.
(unbind_to): Do not copy the binding; not needed, now that we
copy old_value in the one place where the copy is needed.
* fileio.c (Fwrite_region): Use ptrdiff_t, not int, for specpdl count.
* lisp.h (BITS_PER_PTRDIFF_T): Remove; no longer needed.
(union specbinding): Rename from struct specbinding.  Redo layout
to avoid the need for 'ptrdiff_t nargs : BITS_PER_PTRDIFF_T - 1;',
which is not portable.  With Sun C 5.12 32-bit sparc, the
declaration causes nargs to be an unsigned bitfield, a behavior
that the C standard allows; but Emacs wants nargs to be signed.
The overall type is now a union of structures rather than a
structure of union of structures, and the 'kind' member is now a
bitfield, so that the overall type doesn't grow.  All uses changed.
* process.c (Fmake_serial_process): Remove unnecessary initialization.

Fixes: debbugs:14643