libsubprocess: support stdout flow control

[chu11]

Separating this from #6291 as its a different beast and different design, and at the moment not an issue hit by users

[chu11]

note some conversation from #6281 and the overflow output callback in remote_output_buffered()

[garlick]

See flux-framework/rfc#427 for a proposed protocol.

[chu11]

Was pondering this and the need.

• with the unbuf support, it isn't needed for a large number of use cases now (but would be needed for flux-exec which doesn't use unbuf)

• without this support, we probably gotta keep the buffer sizes large at 4 meg libsubprocess: decrease default buffer size #6385

I think we care about #6385 a lot b/c 4 megs is a good chunk of memory or people are launching alot of processes