BackupPC storage and pooling code modules, a C-Code Perl extension

This Package is needed by BackupPC Server on the Backup server machine.

Significant parts of the BackupPC storage and pooling code have been written in C (the same code is used in the server rsync_bpc). BackupPC::FileZIO, BackupPC::PoolWrite, BackupPC::Attrib, BackupPC::AttribCache and BackupPC::PoolRefCnt (reference counting and storage) are all replaced with BackupPC::XS, a C-code perl extension.