From "UNIX, version 2.0" to "UNIX, version 3.0". This is the value used
by info-zip, ark and file-roller (on xubuntu). The others I tested
file-roller (on ubuntu) and 7z uses "UNIX, version 6.3".

Using "3.0" seems to be a safe bet.

This is an update for Stuk#200.

If we don't keep the original unix/dos permissions, this will lead to
hard-to-debug issues, where the permissions are not exactly like they
were specified.
This commit changes that and uses the exact file mode. It also updates the
default umask from 022 to 002 (with unixPermissions == null, files will
get 0664 and folders 0775).

To put a folder with custom permissions, folder() is not a good
candidate : its behavior is to re-use existing entries. Instead, this
commit documents the existing dir attribute.