dsh: add new write command to save list to file if user forgot to start with --output about mpifileutils HOT 4 OPEN

When using the rm command in dsh, the list of files is updated. It would be nice to write this to an output list so someone can pick up where they left off if they stop and run dsh again at a later time.

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This is now supported if the user also specifies -o when starting drm.

Let's addionally add a "write filename" command to let a user write to a file if they forgot to start drm with -o.

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@Sikich, this task would be a good intro to the guts of dsh for you. This is really an experimental tool that I think could be really useful, but I'm not sure we want to fully support it on all systems (hence experimental). Anyway, it seems to work well here so far.

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The -o and --file options are good for v0.7. The write command can come in a later release.

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