shmem_pe_accessible needs to check for MPMD

[jeffhammond]

The source TODO comment describes this issue succinctly.

/* 8.3: Accessibility Query Routines */
int shmem_pe_accessible(int pe) 
{ 
    /* TODO: detect MPMD launching, i.e. if PE is running same binary as me.
     *       MPI_APPNUM attribute of MPI_COMM_WORLD (MPI-3 10.5.3) is the way.
     *       Create a window containing these values so that any PE can Get it
     *       and compare against the local value. */
    return ( 0<=pe && pe<=shmem_world_size ); 
}