Support for optionally running http server as non-root

[maglar0]

I don't know best practices etc on how to deal with things like this, so maybe this change is not the right way forward, but it does seem to work to run part of it as non-root. From what I gather when googling, it is impossible to run smartctl as non-root, so that part is still run as root (the only part run as root).

[matusnovak]

Hi @maglar0

The changes look good to me, but what is the use case to run a non-root version of this exporter?

I understand that there is a general preference to run things like these as user instead of a root (which I completely agree with), but since you are still running part of the script as root, doesn't that violate the idea of these changes?

[maglar0]

There is a flaw in this pull request in that the root part will execute anything you ask it to. So don't merge this. I hope I have time to fix it in the future, but I kinda did changes in a totally different direction after this in trying to not wake up the hard drives if they are in standby mode.

The general idea is to run as little as possible as root, in this case in particular we might not want to run the http parser etc as root, thus minimizing attack surface. But again, my implementation is flawed.