external prrte support for 6.0.x release stream and main

[hppritcha]

Issue to mark fact that we don't plan to use all of the changes in https://github.com/open-mpi/ompi/pull/13299 but just those related to the single-shot launch mechanism. Since we also will want to continue support building against external prrte add some protection for this and also name of dvm launcher command in dpm.c