Prevent open FIFO prom leaking to subprocess

This fix previews window staying open after nnn exits, because they
inherited open FIFO (in write mode) from parent nnn process, and never
close it.

src/nnn.c

@@ -4679,7 +4679,7 @@ static void populate(char *path, char *lastname)
 static void notify_fifo()
 {
 	if (fifofd == -1) {
-		fifofd = open(fifopath, O_WRONLY|O_NONBLOCK);
+		fifofd = open(fifopath, O_WRONLY|O_NONBLOCK|O_CLOEXEC);
 		if (fifofd == -1) {
 			if (errno != ENXIO)
 				/* Unexpected error, the FIFO file might have been removed */