Initial FreeBSD support

[dveeden]

#433

• Recognizes FreeBSD based on cfg data
• Only a few of the functions are implemented yet.

As you may recognize this is mostly based on unknown, but with things like

use crate::DiskExt;

instead of

use DiskExt;

I'm not sure why that's needed but without this it was failing to build with errors like this:

$ cargo b
   Compiling sysinfo v0.17.2 (/usr/home/dvaneeden/sysinfo)
error[E0432]: unresolved import `ComponentExt`
 --> src/unknown/component.rs:7:5
  |
7 | use ComponentExt;
  |     ^^^^^^^^^^^^ no external crate `ComponentExt`

This error happens on an unmodified master branch as well as you can see here.

[GuillaumeGomez]

So overall, this PR is not really useful considering it's simply copying unknown target content. Therefore, I propose you the followings:

• Open a PR for adding the file explaining how to add a new target
• Open a PR to fix the freebsd compilation issue (and add a CI entry to ensure that it doesn't regress again)

[dveeden]

What is the minimal functionality that needs to be present for you to merge this?

[GuillaumeGomez]

Support for most functionalities. If something cannot be supported, (like Components on Windows), it needs to provide an explanation so that it can eventually be added in the future.

[GuillaumeGomez]

Closing in favor of #620.