OpenPMIx fills in a variety of info when it detects that we are in
singleton mode. Best way of detecting it is to look for the "singleton"
at the beginning of the returned nspace.
Make the modex recvs optional so we don't bounce up to the server and
then to the host trying to retrieve job-level info that must be given to
us at job start.
Signed-off-by: Ralph Castain <rhc@pmix.org>
674134430c