ptrdiff_t, not ssize_t

ptrdiff_t, not ssize_t

* src/buffer.c (bufmap_reset, _flush_write):
Use ptrdiff_t, not ssize_t, to record pointer differences.
POSIX allows systems where size_t is 64 bits but ssize_t is only 32;
Ultrix used to do that, though no current systems do.