executor: fix max signal/cover filter mapping into subprocesses

There is a quirk related to posix_spawn_file_actions_adddup2:
it just executes the specified dup's in order in the child process.
In our case we do dups as follows:
20 -> 4 (output region)
4 -> 5 (max signal)
So we dup the output region onto 4 first, and then dup the same output region
(fd 4 becomes the output region) onto 5 (max signal).
So we have output region as both output region and max signal.

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