Add support for large size files using pax/posix format

[throx]

Added support for the size attribute being in the pax/posix pseudo-entry in the same way a large filename could be in there. This fixes the problem of a posix compliant .tar file with an entry of >8G failing to parse properly.

I certify that I own, and have sufficient rights to contribute, all source code and related material intended to be compiled or integrated with the source code for the SharpZipLib open source product (the "Contribution"). My Contribution is licensed under the MIT License.