[mpich2-core] Disp_unit in dynamic windows should not be signed

Hi All, The MPI spec states that the disp_unit argument to RMA calls must be a non-negative integer (of type MPI_Aint). For dynamic windows, the displacement is the address relative to MPI_BOTTOM (i.e. NULL), which can be in the negative range when interpreted as a signed integer. We currently implement MPI_Aint as a signed integer. Is this correct? If so, we need to make changes to the RMA chapter to account for disp_units on dynamic windows (e.g. to allow us to ignore signedness of disp_unit for windows of this flavor). ~Jim.