Min left servers

MLdonkey will cancel a servers list cleanup if the number of servers that would remain would be less than min_left_servers.

This is a safety against bad cases or user stupidity (MLdonkey running but that cannot reach any server because of firewalling, black listing of port 4661,...) , but a too high min_left_servers can be harmful too (instead of keeping no servers, MLdonkey can keep an ever growing list of obsolete server addresses !)