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.