FreeBSD support

[vthriller]

• progress works,
• progress -m works,
• progress -p works,
• also seems to be working as usual on linux after these changes.

Based on PR #78. Closes #77.

[vthriller]

(Sorry for force-pushes: I've fixed a couple of segfaults caused by classical copy-paste errors and took that opportunity to re-read the code a day later to make sure I didn't miss anything else. This PR should be fine by now.)

[Xfennec]

First, thanks for this work, it's awesome to get FreeBSD support for this tool.

I'm totally OK to merge such support, but I can't maintain this by myself, so I need to be sure that you're around if a future change implies changes in this part of the code, like @BestPig did in 2015 for the MacOS X port.

What's your opinion on that ?

[vthriller]

Hey, why not, sign me up! While I don't log into FreeBSD machines that often, it's rather easy to spin up in a VM from a prebuilt image, and the rate of changes that this relatively simple project sees should make it quite comfortable to maintain in the future.

[Xfennec]

Nice!

I've reviewed the patch and it's OK as-is for me, but do you want some time before I do the merge to add additional changes?

[vthriller]

It seems to be working fine now, so I think this PR is essentially done.

[Xfennec]

Thanks again for this work!