Fix performance of unpacking zip files.

Fix performance of unpacking zip files.

Unfortunately the zip functions absolutely need
the input stream to be buffered, or performance will
be very bad.
Also the zip read functions return very short reads
(< 1 kB on average), so we need to buffer them up
to get a usable write size.
Using a FileChannel improves performance a little
bit in addition, but it's probably the least
critical change.