Add symlink support to ZIP file creation, and a few tests.

This implementation sets the "version made by" field
(creator_version) to Unix for symlinks, only; regular files and
directories are still marked as originating from FAT/VFAT/NTFS.

Also set "external file attributes" (attr2) to 0 for regular
files and 16 for directories (FAT attribute), and to the file
mode for symlinks.

We could always set the creator_version to Unix and include the
mode, but then Info-ZIP unzip would set the mode of the extracted
files to *exactly* the value stored in attr2.  The FAT trick
makes it apply the umask instead.  Note: FAT has no executable
bit, so this information is not stored in the ZIP file.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <junkio@cox.net>