pf: use time_uptime rather than time_seconds

Use 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 compute the correct wall
clock time when userspace requests the status.

ok bluhm@ mikeb@

Reviewed by: imp
Obtained from: OpenBSD, patrick <patrick@openbsd.org>, 63b24bda99
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D50722