Add ZFS cache awareness to memory meters.

[zielmicha]

Fixes #44

This is #153, but rebased to be mergable with the current head.

[natoscott]

@zielmicha I believe this PR will need to be reworked to fit in with the code from #920

It should simplify the changes needed here immensely - the arcstats file parsing is done and there's no need for configure.ac changes, so most of this PR goes away (put not all AFAICT).

We're creating a community 3.0.0 htop release, it'd be great to have this functionality included for Linux ZFS.
[ https://github.com//issues/992 ]

I notice the FreeBSD code (freebsd/FreeBSDProcessList.c) already handles the memory accounting adjustments you're making here - but from my understanding of the code in both PRs, I believe the functionality you seek here will still be missing for Linux.

Please see https://github.com/htop-dev/htop for a tree with #920 merged - if someone with ZFS filesystems could create a new PR ASAP for the htop-dev repo, it'd be great to include this in that new release. Thanks!