unzip: add -Z for "zipinfo mode".

But don't document it because it's a silly idea. Just call zipinfo
directly if you want zipinfo!

There are multiple uses of `unzip -Z` in the AOSP build, though, so we
may as well support it if people are already using it.

Test: manual
Change-Id: I04b05795badf63febe1210fbeaa96e3bd27237f1