Use a listen(2) backlog of 128.
There's a chance that incoming (legitimate) connections arrive faster than what it takes to spawn a new thread and get back to listen(). Therefore we should ask the stack to queue at least one entry, i.e. use a backlog value of at least 1. There's arguable also a chance of more than two concurrent incoming connections, which would make a case for a backlog value greater than one. A reasonable high value seems to be 128, which also is what SOMAXCONN is on many unix systems. In the choice between 1 and 128, an argument against the higher value is that it may mask the potential problem of spending a long time serving incoming connections. Being reasonably confident that radsecproxy is efficient when it comes to serving incoming connections, by handing them off to a newly spawned thread, I think that 128 is a fine choice. Closes RADSECPROXY-72.