tree-wide: Add a few missing O_CLOEXEC

I noticed an instance of this while working on https://github.com/ostreedev/ostree/pull/861
Which apparently I cargo-culted into the new system generator bits.
Let's break this out as a small concise change.

Closes: #866
Approved by: jlebon

src/switchroot/ostree-prepare-root.c

@@ -54,7 +54,7 @@ touch_run_ostree (void)
 {
   int fd;
 
-  fd = open ("/run/ostree-booted", O_CREAT | O_WRONLY | O_NOCTTY, 0640);
+  fd = open ("/run/ostree-booted", O_CREAT | O_WRONLY | O_NOCTTY | O_CLOEXEC, 0640);
   /* We ignore failures here in case /run isn't mounted...not much we
    * can do about that, but we don't want to fail.
    */