[mpich2-dev] 64-bit MPI_Aint/32-bit MPICH2/ROMIO

[Bob Cernohous]

This is the initial patch for the work I described in my earlier email.

It includes some configure changes to make MPI_Aint 8 bytes.  It has
some basic macros which we'll be using in the code to verify/test 
assumptions and to safely cast datatypes.  And it has some initial code 
changes.

I tried to separate out irrelevant changes in our local tree.  It will be easier when our code base settles on an official MPICH2 release.