* lib/xalloc-oversized.h (xalloc_count_t): Remove; no longer
needed and was evidently error-prone anyway.
(xalloc_oversized): Omit some over-optimization that caused
SIZE_MAX to not be treated as too large (the Gnulib convention) on
unusual platforms where PTRDIFF_MAX == SIZE_MAX. This change
should not affect typical platforms where PTRDIFF_MAX < SIZE_MAX.
When optimizing, simply use ptrdiff_t instead of xalloc_count_t.