uvloop simply disable SIGCHLD?

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* **uvloop version**: 0.14.0
* **Python version**: 3.6.8
* **Platform**: linux
* **Can you reproduce the bug with `PYTHONASYNCIODEBUG` in env?**:
* **Does uvloop behave differently from vanilla asyncio? How?**: uvloop cannot add SIGCHLD handler

I need to monitor child process exit.
```
loop.add_signal_handler(signal.SIGCHLD, handle_child_exit)
```

However, uvloop simply disable the signal with the following error.
RuntimeError: cannot add a signal handler for SIGCHLD: it is used by the event loop to track subprocesses

Is there any solution to support SIGCHLD?

Start with the reported loop.add_signal_handler(signal.SIGCHLD, handle_child_exit) case and reproduce the RuntimeError on Linux with uvloop 0.14.0. Compare the behavior with vanilla asyncio, then determine whether SIGCHLD can be supported without breaking the event loop's subprocess tracking. Done means child-process exits can be monitored through a user handler without that conflict.

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