fix pack/unpack

[jeffhammond]

MPI pack and unpack functions are not count-safe because they use int to express a number of bytes.

ROMIO uses these functions, so I guess @roblatham00 would like to see this fixed.

Pack

int MPI_Pack(const void *inbuf,
             int incount,
             MPI_Datatype datatype,
             void *outbuf,
             int outsize,
             int *position,
             MPI_Comm comm)

Unpack

int MPI_Unpack(const void *inbuf,
             int insize,
             int *position,
             void *outbuf,
             int outcount,
             MPI_Datatype datatype,
             MPI_Comm comm)