[Proposal] Add a helper method to get the Unix file permissions off of ZipEntry

[IvanLH]

Steps to reproduce

1. Zip a file on Linux using Info-Zip
2. Unzip using the library
3. There's no easy or well documented way to retrieve the file permissions whose structure is this: https://unix.stackexchange.com/questions/14705/the-zip-formats-external-file-attribute

Expected behavior

I am proposing to add a method called int GetUnixStyleFilePermissions() to ZipEntry which will get the necessary bits and return them as int.

Actual behavior

No such functionality exists

Version of SharpZipLib

1.2.0

Obtained from (only keep the relevant lines)

• Package installed using NuGet

[piksel]

ZipEntry.ExternalFileAttributes is what you are looking for.