This commit modifies ompi's process list group object to support a
sentinel value for non-existant ompi_proc_t objects. The sentinel was
chosen to be the negative of the opal_process_name_t of the associated
ompi_proc_t. This takes advantage of the fact that on most (all?)
systems the top bit of a user-space pointer is never set. If this
changes then a new sentinel will be needed.
In addition this commit modifies the way ompi_mpi_comm_world is
initialized to fill in the group with sentinel values if the number of
processes exceeds the new add_procs behavior cutoff.
Signed-off-by: Nathan Hjelm <hjelmn@lanl.gov>
