Use _exit() to terminate children

Use _exit() to terminate children

The NNN_PIPE file gets deleted after spawning a child in NOWAIT mode. Steps:
- open `nnn` with option `-x`
- press `;o` (custom binding) to open fzopen plugin
- press `^J` to select a file; it will spawn plugin `.cbcp` in NOWAIT mode
- when the intermediate child quits, the NNN_PIPE of parent is also deleted