broker: add function to pmiutil to fetch taskmap

[grondo]

Problem: The broker can't take advantage of the special flux.taskmap PMI key when PMI_process_mapping is truncated, since it currently only attempts to fetch PMI_process_mapping using pmiutil's broker_pmi_kvs_get().

A new function to fetch and create a struct taskmap should be added to broker/pmiutil.c which can return a taskmap in any of the broker's PMI "modes": singleton, dlopen, wire.1, or pmix. As noted in #4798, and optimization can be used when using the simple_client interface (i.e. wire.1), but the code to fetch and decode flux.taskmap or PMI_process_mapping will have to exposed, since the broker does not use either of the clique functions that result in fetching the taskmap internally.

I'm unsure how the taskmap would be created for the PMIx case.

[garlick]

Is this resolved by #4798 or did we think there was more to do here? Since flux.taskmap is only present in flux's PMI implementation, is there any reason to support those other "modes"?

[grondo]

Right, I think the current implementation is simple enough that we won't gain much by pushing the taskmap fetch into the pmiutil interface. Closing.