socket-util: Move get_null_fd() to daemon.c.

get_null_fd() is only called from daemon.c.
It does not need thread safety features anymore as
it is called either through daemonize_start() or
indirectly through daemonize_complete() once.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>