Comments about 'db' arguments.
Fixes #4205: ensure sizeof(MPI_Count) <= sizeof(size_t)
    
- Move the ptrdiff_t tests up higher in configure.ac to be with the
  rest of the type tests.
- Create new OMPI_FIND_MPI_AINT_COUNT_OFFSET for finding the
  corresponding types of MPI_Aint, MPI_Count, and MPI_Offset.
  Consolidate all the old C and Fortran tests into this new macro (and
  .m4 file).
- Fix Fortran MPI_*_KIND tests that incorrectly keyed off assumed
  types (e.g., int64_t) rather than whatever the corresponding C
  MPI_Aint, MPI_Count, MPI_Offset types turned out to be.
- Add new logic to ensure that sizeof(MPI_Count) <= sizeof(size_t),
  because our entire PML, BTL, and convertor infrastructure requires
  this.  As a side effect, just like MPI_Offset the same type of
  MPI_Count (because MPI_Count has to be able to hold an MPI_Offset,
  so we can't let MPI_Offset be larger than a MPI_Count).