terminal-util: open /dev/null with O_CLOEXEC in make_stdio_null()

Ultimately, O_CLOEXEC should be off in fd 0, 1, 2, but when we open
/dev/null here it's unlikely to be < 0, and after dupping the fd to 0,
1, 2 we turn off O_CLOEXEC explicitly anyway.

Unless we know that what we are about to open will return 0, 1 or 2 we
should always set O_CLOEXEC in order to be safe to other threads forking
of subprocesses at the wrong moment.

src/basic/terminal-util.c

@@ -898,7 +898,7 @@ int make_stdio(int fd) {
 int make_null_stdio(void) {
         int null_fd;
 
-        null_fd = open("/dev/null", O_RDWR|O_NOCTTY);
+        null_fd = open("/dev/null", O_RDWR|O_NOCTTY|O_CLOEXEC);
         if (null_fd < 0)
                 return -errno;