tools/ocaml: Dup2 /dev/null to stdin in daemonize()

tools/ocaml: Dup2 /dev/null to stdin in daemonize()

Don't close stdin in daemonize() but dup2 /dev/null instead.  Otherwise, fd 0
gets reused later:

  [root@idol ~]# ls -lav /proc/`pgrep xenstored`/fd
  total 0
  dr-x------ 2 root root  0 Feb 28 11:02 .
  dr-xr-xr-x 9 root root  0 Feb 27 15:59 ..
  lrwx------ 1 root root 64 Feb 28 11:02 0 -> /dev/xen/evtchn
  l-wx------ 1 root root 64 Feb 28 11:02 1 -> /dev/null
  l-wx------ 1 root root 64 Feb 28 11:02 2 -> /dev/null
  lrwx------ 1 root root 64 Feb 28 11:02 3 -> /dev/xen/privcmd
  ...

Signed-off-by: Christian Lindig <christian.lindig@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>
(cherry picked from commit 677e64dbe315343620c3b266e9eb16623b118038)
(cherry picked from commit 4b72470175a592fb5c0a5d10ed505de73778e10f)