[FIXED JENKINS-19192] LoginCommand failed on Windows after introduction of JNR.

Root cause of class loading error (on jnr.ffi.util.ref.internal.Finalizer$ShutDown) still unclear,
but easy fix is to simplify ClientAuthenticationCache.save to use FilePath.chmod,
which at least for now relies on hudson.util.jna.GNUCLibrary rather than JNR.