pf: use time_t for storing time_t values

No change to the underlying type, so no ABI change.

We define time_t as uint64_t if LP64, otherwise uint32_t,
and only define LP64__ if long is 64 bits.
In other words: __time_t == long.

ok henning@ deraadt@

Obtained from: OpenBSD, guenther <guenther@openbsd.org>, 6c1b69a0ff
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D48963