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.

When I was debugging my setup I thought there were run-away zombie
client connect attempts - until I dug into the code and realized the "5 active!"
was talking about total file descriptor count (not clients).

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)