Kestrel should set O_CLOEXEC flag when listening on socket passed in via file descriptor

Kestrel should set `O_CLOEXEC` flag on socket when listening on socket passed in via file descriptor (e.g., systemd activation). We're currently leaking these file descriptor to child processes.

There is currently no managed API that wraps [`fcntl()`](https://man7.org/linux/man-pages/man2/fcntl.2.html), so we may have to P/Invoke in Kestrel to achieve this behavior.

Related: https://github.com/dotnet/runtime/issues/46832