sort: prefer xpalloc to x2nrealloc

sort: prefer xpalloc to x2nrealloc

* src/sort.c (struct buffer, temp_dir_count, temp_dir_alloc)
(create_temp_file, add_temp_dir, fillbuf):
Prefer idx_t to ptrdiff_t/size_t for nonnegative directory counts.
(add_temp_dir, fillbuf): Use xpalloc, not x2nrealloc.
* src/system.h (X2NREALLOC): Remove; no longer used.