Performance optimization, code clarification, and bug workaround.

Performance optimization, code clarification, and bug workaround.
When reading the bodies of Zip archive entries, request a minimum of 1
byte, rather than a minimum of the full entry size.  This is faster
(since it does not force the decompression layer to combine reads) and
works around a bug in the "none" decompression handler (which I'm
testing a separate fix for now).  I've also renamed "bytes_read" to
"bytes_avail" in several places to more accurately reflect that the
value returned from (a->compression_read_ahead) is the number of bytes
available, not necessarily the number of bytes requested.