add --version argument to ecryptfs-recover-private

[Karl-Philipp Richter]

Add a --version argument to ecryptfs-recover-private so that version of the script or of the ecryptfs-utils version can be retrieved easily on command line (should be possible to invoke without root permissions)

This makes also sense if you consider ecryptfs-recover-private as a script because it might become a program one day and divergate from ecryptfs-utils versions.