stdcountof-h: always return size_t

stdcountof-h: always return size_t

* lib/stdcountof.in.h (__SIZE_TYPE__): Define to size_t,
including stddef.h to get it, if not already defined.
This avoids a bit of namespace pollution.
All uses of size_t changed to use __SIZE_TYPE__.
(countof): Return size_t, even if size_t is narrower than int (!).
While we’re at it, simplify ‘sizeof ((a)[0])’ to ‘sizeof *(a)’
as it’s simpler and later code uses the ‘*(a)’ notation already.