Fixes #2904

The intercomm "merge" function can create a linkage between procs that was not reflected anywhere in a modex, and so at least some of the procs in the resulting communicator don't know how to talk to some of the new communicator's peers.

For example, consider the case where:

1. parent job A comm_spawns a process (job B) - these processes exchange modex and can communicate

2. parent job A now comm_spawns another process (job C) - again, these can communicate, but the proc in C knows nothing of B

3. do an intercomm merge across the communicators created by the two comm_spawns. This puts B and C into the same communicator, but they know nothing about how to talk to each other as they were not involved in any exchange of contact info. Hence, collectives on that communicator now fail. 

This fix adds an API to the ompi/dpm framework that (a) exchanges the modex info across the procs in the merge to ensure all procs know how to communicate, and (b) calls add_procs to give the btl's a chance to select transports to any new procs.

cmr:v1.7.3:reviewer=jsquyres