devstat on systat -zarc and -swap pages

[ota_j.email.ne.jp]

Summary

Consolidate devstat piece and share I/O related pages, -vm, -zarc, and -swap pages.

Ref - https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=213310

[ota_j.email.ne.jp]

Removed some of experimental code.

[ota_j.email.ne.jp]

Thanks.
A couple of month delay is a minor.
I just started thinking to add few more enhancements.
It will be nice if you pick up next one, too ( coming in a couple of months, too).