curvetun: Enhanced server connect / disconnect syslog messaging

Made the messages a bit clearer for the average user, who may
be unaware (or not care) that there are four file descriptors
always open in addition to the number of active connections.

Old connect/disconnect messages:
curvetun[25448]: New connection from 68.107.99.106:41208 with id 15 on CPU0, 5 active!
curvetun[25448]: Closed connection with id 15, 4 active!

New connect message:
curvetun[16247]: New connection from 68.107.99.106:36581 (1 active client connections) -  id 15 on CPU0
curvetun[16247]: Closed connection with id 15 (0 active client connections remain)