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
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