BSD `signal(2)` semantics make some system calls (e.g. for `write`)
restartable when interrupted by a signal handler. Use `SA_RESTART` to
enable these semantics everywhere that supports them.
This is required by C++ stream libraries that interpret `EINTR` as any
other error, set `badbit`, and stop writing. I've observed this with
`libstdc++` during a `std::cout.flush()` call interrupted by `SIGCHLD`.
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