Fix truncated output when a command exits

[dcalvo]

Follow-up to #98. Fixing the :epipe crash uncovered a second problem on the
same path: output gets dropped and nothing tells you about it. The crash was
happening first, so it hid this. I'm using MuonTrap to run commands in an agent
harness and feed the output back to a model, which has no way to tell a
truncated result from a complete one.

process_stdio() only forwards as much as the flow control window allows, and
child_wait_loop() stops polling the output fds once that window is empty. When
the child is reaped we return right away, so anything still sitting in the pipes
goes away with it. That's why what gets lost is always a whole number of
windows.

cmd/3 just hands back a short binary with a 0 exit status, so there's nothing
to key off of. Two commands, 40 runs each:

• print_and_exit.test, 66890 bytes: 0/40 runs came back complete, worst case
  kept 10240 bytes
• seq 1 100000, 588895 bytes: 24/40 complete, worst case kept 542720 bytes

The child's writes have all finished by the time it exits, so draining the pipes
before returning is enough. Polling with a 0 timeout tells us when a pipe is
empty, so the usual case costs one extra syscall. If the window is empty I wait
on acks the same way the exit path already does.

The test I added fails at 461/1001 lines without the C change. Related:
test/print_a_lot.c has had a sleep(1) in it since flow control landed in
d3d0336, commented "muontrap doesn't wait for all output to be consumed". That
sleep is why the existing "prints a lot" tests pass, so I left it alone and used
print_and_exit.test instead.

Only tested on aarch64 Linux, so the splice() branch. The read/write fallback
goes through the same drain loop but I have no way to run it here. cgroup tests
are excluded since I didn't set up the muontrap_test cgroup; the rest pass.

[fhunleth]

Thanks for sending his up. I really appreciate the time you're putting into making this use case work. The CI failure is unrelated. The code update looks good on quick skim. My plan is to put in some time on muontrap next week and I'll merge your PR then with that pass.

[dcalvo]

Thank you for your work! I have found your project extremely useful both personally and professionally, just doing what I can to contribute back :)