Make sure that time advances in a daemon between calls to time_refresh().

Make sure that time advances in a daemon between calls to time_refresh().

Open vSwitch uses an interval timer signal to tell it that its cached idea
of the current time has expired.  However, this didn't work in a daemon
detached from the foreground session (invoked with --detach) because a
child created with fork() does not inherit the parent's interval timer and
we did not re-set it after calling fork().

This commit fixes the problem by setting the interval timer back up after
calling fork() from daemonize().

This fix is based on code inspection (which was then verified to be correct
through testing).  It may not fix any actual problems in practice, because
time_refresh() is called every time through the poll loop, and the poll
loop typically runs more quickly than the periodic timer fires (1 ms or so
average in ovs-vswitchd, vs. 100 ms timer interval).