systat: Implemented per-process swap display on -swap

[ota_j.email.ne.jp]

Summary

Use libprocstat to calculate current swap space usage per process
and display them in usage order.

This contains changes from D29337.

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=254723

Test Plan

% systat -swap

Please check https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=254723

[bcr]

A few small corrections to the man page.

usr.bin/systat/systat.1

286

This sentence is just a listing with no verb.

289

s/objecte/object/
s/objects/object/

[mckusick]

Not having seen this review, I started a review at https://reviews.freebsd.org/D29754. I am fine with abandoning my review though I do think you should consider incorporating my version of the manual page.

[ota_j.email.ne.jp]

We have a duplicate review for this and are tracking changes in https://reviews.freebsd.org/D29754.
The comments are addressed in https://reviews.freebsd.org/D29754.