Using alternatives to manage postgis.control

[Christoph Berg]

Re: To Bas Couwenberg 2019-07-16 <20190716120148.GE10017 at msg.df7cb.de>
> I'll do some clean up of the rule files; currently I'm working on
> making the pg-postgis-3 package actually depend on
> pg-postgis-3-scripts as getting that installed without the dependency
> is really painful, and bad UX. I hope that using "alternatives" for
> the *.control files does the trick. Will push a branch for review.

I pushed an "alternatives" branch. Hopefully I got the alternatives
and diversions handling right...

Christoph