port "cask" -- installing prebuilt binaries

[Lothar Haeger]

> Am 05.08.2020 um 14:28 schrieb Ken Cunningham <ken.cunningham.webuse at gmail.com>:
> 
> I wasn't imagining a variant for binary-only installs.

That's just an idea on how this could be presented to users. Also an easy way to identify ports that are basically binary redistribution (`port list variant:prebuilt`). Also would allow to provide a single port both ways, and an easy way to configure user preference via existing mechanisms.

> If a port can be built, we should build it.

Sure, let's repeat this a few more times until everyone *really* got that. :-) 

> But for things we can't build, (like Zoom or Microsoft Word) installing a binary is the only option and some ports of this type seem to be what people are submitting recently.

There may be ports where providing it both ways makes sense (e.g. some ports may not build on certain macOS/XCode versions but on others), though, so if we can find a way to provide both in parallel, that could help.

> This is fine -- some work to maintain -- but I was just wondering if we might coalesce around a scheme to make it obvious somehow to users that these were different animals than our usual ports.

I totally agree with this, just think this is merely a starting point to optimize creation and handling of such ports.