Doing that will break subprocesses.

To clarify this ticket: I'm talking about prohibiting `loop.add_signal_handler(SIGCHLD)` as it would break child processes watchers that asyncio installs. In other words, setting a custom SIGCHLD breaks asyncio internals.

We can allow user-set SIGCHLD signals if we add some code to make sure that 'add_signal_handler(SIGCHLD)' delivers the signal to *both* asyncio internals and user code.  But I'm not sure if there's a valid use case for listening for SIGCHLD for user code at all.

For now, I disabled SIGCHLD in uvloop, let's see if people complain.

Some people may have reasons to do that (for example: they write their own subprocess implementation). I suggest emitting a warning rather than forbidding it.