libz: Use off_t instead of long.

libz:  Use off_t instead of long.

This problem was triggered by clamav.  As the comment in zconf.h states,
we'd prefer to always define Z_HAVE_UNISTD_H, but libstand has some issues
with this, which is why we originally had the change to the vendor
source to include <unistd.h> in gzguts.h.

While we're here, there's no point in defining HAVE_MEMCPY in
Makefile.stand, since it's already defined elsewhere in zconf.h.