systat: use and correctly display 64bit counters.

Following struct vmtotal changes, make systat use and correctly
display 64-bit counters. Switch to humanize_number(3) to overcome
homegrown arithmetics limits in pretty printing large numbers. Use
1024 as a divisor for memory fields to make it consistent with other
tools and users expectations.

Submitted by: Pawel Biernacki <pawel.biernacki@gmail.com>
Sponsored by: Mysterious Code Ltd.
PR: 2137
Differential revision: https://reviews.freebsd.org/D13105