Use a different UDP discovery interval if the tunnel is established.

Use a different UDP discovery interval if the tunnel is established.

This introduces a new configuration option,
UDPDiscoveryKeepaliveInterval, which is used as the UDP discovery
interval once the UDP tunnel is established. The pre-existing option,
UDPDiscoveryInterval, is therefore only used before UDP connectivity
is established.

The defaults are set so that tinc sends UDP pings more aggressively
if the tunnel is not established yet. This is appropriate since the
size of probes in that scenario is very small (16 bytes).