Delegate Control-Z handling (SIGTSTP)

The [Control-C handling on Unix](https://hackage.haskell.org/package/process-1.4.2.0/docs/System-Process.html#g:4) documentation describes a `delegate_ctlc` option that allows the child process to control what happens when Control-C is pressed. I'm wondering if a similar option should be provided to allow the child process to control what happens when Control-Z is pressed.

Without such an option, the child process can install a SIGTSTP handler to customize Control-Z behavior, but the parent process suspends unconditionally, immediately returning control to the shell. This would be undesirable for applications that need to react differently to Control-Z (e.g. by suspending an internal computation instead of suspending the whole program).

A `delegate_ctlz` option would cause the parent process to ignore SIGTSTP while waiting for the child process to _either terminate or suspend_ (using the WUNTRACED option). If the child suspends, then the parent should suspend itself in turn.

FWIW, the [“Proper handling of SIGINT/SIGQUIT”](http://www.cons.org/cracauer/sigint.html) article doesn’t mention anything about SIGTSTP.

Read the existing delegate_ctlc documentation and compare its Unix signal behavior with the proposed delegate_ctlz option. Investigate how the parent waits for termination or suspension with WUNTRACED; done means the child can handle SIGTSTP and the parent suspends itself only when the child suspends.

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