fix: incorrect manner of terminating the rsync process

[machinly]

现有代码中 kill rsync 的方法 kill $(ps -o pgid=" + std::to_string(pid) + ")" 有问题。

ps -o pgid=12345 的含义是列出的活动进程组ID，并把 header "PGID" 改为 12345。

kill 掉这个命令返回结果会 kill 所有当前所有的活动进程。

这个 PR 将其修改为 kill -- -$(ps -o pgid -p " + std::to_string(pid) + " | grep -o '[0-9]*')"，

意图为：

列出 pid 为 12345 的进程的进程组 id ps -o pgid -p 12345，并过滤掉非数字结果，得到所需的进程组 id 666。

然后 kill 掉进程组 id 为 666 的所有进程 kill -- -666。

[cr-gpt]

The reviewed code is a patch for stopping an rsync process. The changes made in the patch include modifying the command used to stop the process by using the pgid (process group id) instead of the pid (process ID). This allows all child processes spawned by the rsync process to be stopped as well.

Improvement suggestions:

• It's recommended to add error handling for the system() call, so any failure can be properly reported.
• Instead of using system(), it's better to use fork()/exec() and waitpid() functions for better control over the execution of the command and avoiding potential security issues.
• It's also good to perform null checks on the returned pointer value of std::to_string(pid) to avoid runtime errors caused by invalid or uninitialized inputs.

[AlexStocks]

我在mac上测试确实没啥问题，在 centos 上测试也没问题