Return time_uptime as value for when pf was enabled instead of

Return time_uptime as value for when pf was enabled instead of
time_second.  Since time_second changes depending on the wall-
clock time, time_second is not a reliable source for the status.
We can even end up with a negative time delta.  Thus, use the
monotonically growing time_uptime and export it to userland.

ok bluhm@ mikeb@