pf: use time_t for storing time_t values

[kp]

Summary

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 <[e-mail]>, 6c1b69a0ff
Sponsored by: Rubicon Communications, LLC ("Netgate")