Feature: libnfs-cp should use NFSv4.2 DEALLOCATE+SEEK to to accurately replicate sparse files

[lionelconswork]

Feature request: libnfs-cp should use NFSv4.2 DEALLOCATE+SEEK to to accurately replicate sparse files

This has been become very important to deal with sparse files, as not all platforms (i.e. embedded, or no NFSv4.2 kernel support) have a NFSv4.2 client readily available.