Make ZipSharp work on Windows

Make ZipSharp work on Windows

This patch adds the zlib functions needed by ZipSharp to the VS
MonoPosixHelper.def export file.

This patch also changes the ZipSharp code to use different code paths
depending on the size of the C long type on the underlying platform. On
gcc/clang the C long type follows the bitness of the targeted architecture,
it's 32-bit on 32-bit systems and 64-bit on 64-bit systems. With the VS
compiler however, the C long type is always 32-bit regardless of the target
architecture. zlib and minizip uses C long in a number of different function
signatures and structs.