Idea: changing libuv not to install a signal handler

首先檢視 libuv/libuv#4179 中提出的變更，以及 uvloop 目前如何使用 libuv 處理子程序與訊號。將該行為與 Python 的 child watchers、Linux 上的 pidfd 支援，以及其他地方使用的專用等待執行緒進行比較；只有在確定安全的整合設計並具備適當的測試後，才算完成。

Hi,

So I've been having problems with a python-uvloop process which is run from a container and spawns child processes. If those child processes spawn their own children which lose their parents (eg. via demonize()) then there's no way to reap them (as PID1 is honour-bound to do). I had been addressing this by patching libuv to do something analagous to FastChildWatcher - ie. reap all process in SIGCHLD handler. But this isn't really safe and can interfere with other libraries or modules (eg. a synchronous process start in another python module).

In the mean time, Python itself has deprecated child-watchers and is now using pidfd on Linux, and a dedicated wait() thread in all other cases.

This means that PID-1 programs can just install their own SIGCHLD handler to reap orphaned processes.

I was wondering if my proposal over on libuv/libuv#4179 might be of interest here? The idea would be to pass a special flag to libuv to tell it not to install a signal handler, and then use the python child watchers to handle reaping of libuv spawned processes.

Without this, a python process which sets up a SIGCHLD handler is going to break if switching from asyncio to uvloop (ie. the switch is no longer transparent, as of python 3.12)