Don't print device statistics when exiting tinc.

Much more detailed statistics are now kept per node, which can be queried at
any time, which makes the device statistics obsolete.
Allow running without ECDSA keys If ExperimentalProtocol is not explicitly set.

To make upgrading less painful, allow running tinc 1.1 without ECDSA keys
unless ExperimentalProtocol is explicitly set to yes.
Give full path to unconfigured tinc-up script.
Don't print an error when no ECDSA key is known for a node using the legacy protocol.