Fixes #3968: Move r29858 to v1.6 branch (fix mpi_status_size for -i8 case)

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Custom patch on ticket based on:

r29858 [[BR]]
Per thread starting here:
http://www.open-mpi.org/community/lists/users/2013/10/22882.php, fix
the value of MPI_STATUS_SIZE for the -i8 case.  Thanks to Jim Parker
for bringing up the issue and providing the patch.

Separate patches are required for v1.6 and v1.7 (and will be attached
to their respective tickets), because this breaks ABI, so we need a
non-default configure option to fix the issue but knowingly break ABI.

cmr=v1.7.4:reviewer=bosilca:subject=Fix MPI_STATUS_SIZE for -i8 case
cmr=v1.6.6:reviewer=bosilca:subject=Fix MPI_STATUS_SIZE for -i8 case
Fixes #3285: Remove debug message in opal_datatype_unpack.c