Fix for a DoS attack:

 A remote user could telnet to the tinc daemon and type only this line:
 61 6 00000000/00000000:28f
 This would deny any packets to be sent to other tinc networks (except
 for to the hosts that run tincd's themselves). Solution is to skip
 hosts in lookup_conn() that have not been activated yet.
Fixed potential conn_list table corruption:
 If a new connection is accepted but a connection with the same subnet
 would already exist in the connection list, the OLD connection is
 terminated.