r24843: Add a "validate-cache" control message to winbindd.

r24843: Add a "validate-cache" control message to winbindd.
So there is a new subcommand "smbcontrol winbindd validate-cache" now.

This change provides the infrastructure:
The function currently returns "true" unconditionally.
The call of a real cache validation function will be incorporated
in subsequent changes.

Michael