explicitly disable gz initalization for _Stream’s used in aux files

explicitly disable gz initalization for _Stream’s used in aux files

The process of writing an auxiliary (index, info) file differs
drastically from that of tar archives: Since files are not added
individually, the encryption must be initialized externally and
the compression layer cannot rely on being enable in the ctor
because, obviously, the latter is executed before the manual
encryption setup can be performed.

Extend the API of the _Stream ctor with a parameter to “noinit”
to request that all initialization be postponed until the
encryption has been set up. This seems to do the trick but is
quite ugly.