More judicious use of file descriptors

Currently execnet uses pipes without the CLOEXEC flag. There should be something like this around the parts that make the pipes:

old = fcntl.fcntl(fd, fcntl.F_GETFD)
fcntl.fcntl(fd, fcntl.F_SETFD, old | getattr(fcntl, 'FD_CLOEXEC', 1))

Sorry for incredibly-lousy-without-testcase bug report, I'm just at the end of my wits after figuring out my app is littered with subprocess.Popen without close_fds=True 😤

未指定文件或测试名称。定位创建管道的 execnet 代码，然后检查相关的 subprocess 或描述符处理测试（如果存在）。当相关管道使用 CLOEXEC 标志，并且回归测试验证预期的描述符行为时，即表示完成。