Improve unzip's handling of stream ZIP files

Improve unzip's handling of stream ZIP files

Search harder for the ZIP magic numbers at the end of a file by checking
16 KiB from the end instead of just 1 KiB.  ZIP files with long comments
(such as certain cryptographically signed files) or those sitting in a
wrapper could have more than 1 KiB of data after the magic numbers, so
they couldn't be read.

Signed-off-by: Dan Fandrich <dan@coneharvesters.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>