Fix / revamp mpirun --output-filename

[jsquyres]

This issue started with #7095. However, it has grown to encompass several things, so I'm opening a new issue to gather them all into one spot.

The initial issue is that mpirun's --output-filename behavior no longer matches what is described in the mpirun(1) man page. Specifically: it doesn't just output to a single file per process any more; mpirun now creates a directory for each MPI process and outputs a stdout and stderr file in there.

1. We should probably rename this behavior to be --output-directory.
   • The new behavior needs to be documented: output to DIR/JOBID/rank.N/stdout|stderr
   • Can also be combined with --merge-stderr-to-stdout (right? test this...)
   • Be sure to mention :nojobid (omit the job ID in the directory hierarchy) and :nocopy (don't also emit to stdout/stderr), and that they can be combined into a single comma-delimited list
   • Mention efficiency of :nocopy (i.e., no IOF used to send back to mpirun) -- probably nearly as efficient as the app writing to its own local files...?
2. BUG FIX: If you specify an invalid suffix (e.g., :noooooocopy, the user is not notified).
3. Need to think through how to use this feature over time from user's perspective
   • What to do in v2.x
   • What to do in v3.0.x (behavior changed compared to v2.x)
   • What to do in v3.1.x
   • What to do in v4.0.x (?added -output-directory / deprecated --output-filename?)
   • What to do in v5.0.x (?--output-directory only?)
4. Make sure to mention killing/deprecating --output-file in NEWS
   • ...unless the old --output-file behavior is resurrected / preserved...? That's an option, if someone wants to do it.

[ggouaillardet]

What does nocopy does?
If it only writes to files but not stdout/stderr then we still need to involve IOF since the MPI tasks might be running on nodes not mounting the filesystem mpirun is writing too.
For performance improvements, I think we would need an other option such as noforward and this should probably not be the default behavior.

[jsquyres]

Hey @rhc54: did you fix the issue that a user is not notified if they specify an invalid suffix?

[rhc54]

Yes, I did

[jsquyres]