Multiple issues with symlinks

[voidc]

While debugging an issue with zips containing symlinks I noticed a couple of things:

• The behavior of mz_zip_writer_add_file and mz_zip_writer_add_path is inconsistent. The latter skips symlinks if store_links and follow_links are false while the former does not. This makes mz_zip_writer_add_file effectively behave like if follow_links were true.
• Attributes are always determined with lstat regardless of follow_links. If follow_links is true, I would expect the writer to store the attributes of the symlink target. Currently, we get zip entries with the contents of the symlink target that also have the ISLINK attribute set. This causes errors when reading such zips, because the reader tries to create a symlink treating the file contents as target path.
• If both store_links and follow_links are set, we get store_links behavior and follow_links is ignored.