daemon: Close standard file descriptors when daemonizing.

daemon: Close standard file descriptors when daemonizing.

Before SSH terminates, it waits for the PTYs that it creates for use as
stdin, stdout, and stderr to be closed.  When any of the Open vSwitch
daemons were started in the background over an SSH session, they held
those file descriptors open and thus the SSH session hung.  This commit
fixes the problem by closing those file descriptors, allowing SSH to
terminate.