Fix mistakes in commit d45099425eb19e420433c9d81d354fe585f4dbd6.

Robert Haas <rhaas@postgresql.org>

Commit: da505eafca81bc81c34f729521a42a093408cb92
Author: Robert Haas <rhaas@postgresql.org>
Date: 2022-01-28T14:02:18Z
Releases: 15.0
Fix mistakes in commit d45099425eb19e420433c9d81d354fe585f4dbd6.

I intended to include a change to the "skip" count in the test
case, but it didn't get folded into the commit. Do that now,
so that non-zlib builds don't break.

The new file bbstreamer_gzip.c needs <unistd.h> to avoid
complaints about dup() not having a prototype, as per buildfarm
returns.

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