refactor/rewrite mpirun to not exec a second executable

[hppritcha]

As part of the Open MPI fork of PRRTe migration into the Open MPI source tree proper, it would be desirable to rework Open MPI's mpirun tool to not exec the prterun executable.

One option would be to export a function(s) from libprrte.so (which may be renamed as part of this project) that the existing mpirun.c could use to replace the need to fork/exec prterun. This approach would require effectively turning the main function in https://github.com/open-mpi/prrte/blob/master/src/tools/prte/prte.c into a library function(s) provided in libprrte.so.

[bosilca]

Can't we skip the fork and just exec the prterun in the mpirun process ?

[hppritcha]

Can't we skip the fork and just exec the prterun in the mpirun process ?

actually mpirun just execv's prterun.