zlib: Properly clean up static/shared distinction

This improves what commit

    e999def1 zlib: clean up static/shared distincion

described as "kind of a mess" and "confusing". And indeed it was confusing.

Now, the concept whether or not the .a file is moved to a split output
is controlled by a clean variable.

The defaults remain unchanged.

The new approach also finally cleanly allows building statically but NOT
using a split output, like all other autoconf-based projects in nixpkgs do
(using the `dontDisableStatic` setting).
That is important for overlays that want to enable static libs for all
packages in one go, without having to hand-patch idiosynchrasies like zlib
had until now.

Until now, if you wanted the .a in the main output, the only way was to go via
`static=false, shared=true` -- which made no sense, because you had to say
`static=false` even though you want a static lib. That is fixed now.

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