pressure-vessel: Avoid sitting on original stdout file descriptor
See https://github.com/ValveSoftware/steam-runtime/issues/593 for bug report.
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pv-adverb: Use an autofreed array to implement pass_fds
The code gets simpler if we always allocate and free this, with it owning the fds directly.
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pv-adverb: Add --assign-fd
We'll use this in pv-wrap in a subsequent commit.
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pv-wrap: Let pv-adverb restore our original stdout/stderr
This allows us to pass a harmless stdout/stderr to bwrap, which will hold them open in its supervisor process, preventing processes inside the container from signalling EOF on those file descriptors.
In particular, this lets a caller use
pv-wrap --launcher -- --info-fd=1 --bus-name=...and read from fd 1 until EOF to detect that launcher-service is ready, without bwrap holding the fd open and therefore preventing EOF detection.
A side benefit of this is that if we opened a structured logging file descriptor as our stdout/stderr, then pv-wrap (and not the game itself) will be "blamed" for any messages that bubblewrap emits from its stdout/stderr - which seems reasonable, since pv-wrap is using bubblewrap as an implementation detail.
Solution for https://github.com/ValveSoftware/steam-runtime/issues/593