Revise the way we handle hetero nodes as users are finding this (a) a significant surprise, and (b) confusing as to when it is required. So try to automate it a bit by creating a topology "signature" that mpirun can share on the cmd line with the remote daemons, thus allowing them to check to see if they match. This isn't comprehensive of course - for now, it only checks the number of each type of hwloc object on the node. This is good enough to pickup major differences (e.g., where we have different numbers of sockets or assigned core bindings).

Retain the hetero-nodes flag for those cases where the user *knows* that there are differences and our automated system isn't good enough to see it.

Will obviously require further refinement as we find out which variances it can detect, and which it cannot.
libfabric: add optional PSM provider.