When you try to execute a file directly after you've written to it,
you might, on some systems, get a "text file busy" error.
So we unfortunately have to sleep to avoid it.
See #8021 for where this was added,
537b3f6cb1 for the same problem.
This cleans up some exit code processing. Previously a failed exec
would produce exit code 125 unconditionally, while a failed posix_spawn
would produce exit code 1 (!).
With this change, fish reports exit code 126 for not-executable, and 127
for file-not-found. This matches bash.
