pf: use time_to for timestamps

Use time_t rather than uint32_t to represent the timestamps. That means
we have 64 bits rather than 32 on all platforms except i386, avoiding
the Y2K38 issues on most platforms.

Reviewed by: Zhenlei Huang
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D36837