Mate and PiTop experimental support

[RogueM]

this is a fairly large update that aims at supporting Ubuntu Mate and Pi-Top OS on the Pi in all ad-hoc scripts (that would be all, except unicornhat, propellerhat and iotphat).

Everything was tested on all 3 relevant OSes: Raspbian (Jessie, with latest 'Pixel' update), Pi-Top OS (image from last June, most recent AFAIK), and Ubuntu Mate (image available following link from RPF, based on Ubuntu 16.04).

[RogueM]

technically, supporting the desired OSes is done in the following manner:

• raspbianonly acts as a first level gate, so setting it to "no" is required for support beyond Raspbian
• osreleases is a list of all supported OSes, using /etc/releases as filter, and exceptions are handled separately (specifying Raspbian in that list is optional but is clearer, so recommended)
• oswarning is a list of all experimental OSes, handled as above. Those will be flagged as 'experimental' to users, but install will otherwise be functionally the same as on Raspbian
• osdeny is a list of all OSes that should be disallowed install when the raspbianonly gate has been relaxed. "Darwin" in osdeny list replaces the old macosxsupport flag (this may change when I start looking again at the particular of OSX)

so, technically, as long as you catch an OS, and - the hard part - distinguish it from the main distro it might be based on, it's a relatively simple system to choose to allow, allow-with-warning, or deny install on a particular OS.

That said, I expect exceptions to catch OSes to be required in more cases than desirable going forward as it seems many of these so-called 'distributions' don't take great care to be uniquely identified via os-release, despite sometimes being significantly distinct at low-level compared to Debian, Ubuntu, or Raspbian.

[RogueM]

Additional Note: by default "Debian" and "Ubuntu" are part of oswarning, which means that there's a fair chance that there are some distros out there that will get through.

That is by design, as I'd like to know if anyone is trying to run the scripts on a distro I am not aware of, where there might be a demand for oli. I don't think any of the scripts can really wreck a distro, worst case expected is that the product does not work (without added work) post-install. Those are marked 'experimental' and flagged as such anyhow, so users should hopefully pause and reflect before applying.

Mate: currently flagged as 'experimental' though it is more a manner of not committing to official support right away, all scripts tested and working. Could be moved to osreleases list quite rapidly, though the only effect would be to remove the warning for users - preferred in the long run though.

PiTop: currently flagged as 'experimental'. Right now they don't have their own repo and my guess would be that when they do / if they do, there will be a few spanners thrown in the works. So will need to see where the situation evolves on that front before (carefully) considering moving "PiTop" into the osreleases list.

Kali: currently only the blinkt script is open to it, otherwise it will be denied. I'm not currently hopeful that many others will make sense to open to that distro.