lua: subprocess: tunnel stderr through mp_log

Pretty much a fringe-feature, but also it's awkward if something appears
on the terminal with no indication for the source.

This is made quite awkward by the fact that stderr and stdout could be
closed at different times, and that poll() doesn't accept "holes" in its
FD list. Invalid (.e.g negative) FDs just make it return immediately, as
required by the standard. So sparse_poll() takes care of the messy
details.

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