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

src/nnn.c

@@ -1302,7 +1302,7 @@ static pid_t xfork(uchar flag)
 			p = fork();
 
 			if (p > 0)
-				exit(0);
+				_exit(0);
 			else if (p == 0) {
 				signal(SIGHUP, SIG_DFL);
 				signal(SIGINT, SIG_DFL);
@@ -1314,7 +1314,7 @@ static pid_t xfork(uchar flag)
 			}
 
 			perror("fork");
-			exit(0);
+			_exit(0);
 		}
 
 		/* so they can be used to stop the child */