Added listen_queue_size as configuration option (Vadim Antipov, Kaspersky Lab)
Notes from Vadim:
1) While using nrpe as a checker proxy on several Linux hosts in our
company we have noticed problems under high check rate. We have found
the problems were due to a small socket backlog size. This value (5)
is hardcoded in nrpe.c now. So, I propose a new configuration parameter
"listen_queue_size". The change is backward compatible.
2) Minor debug print errors were fixed.
"Connection from %s closed." debug message was broken for allowed IPv4 hosts.