MINOR: server: add a list of private idle connections

For now it's not populated but we have the list entry. It will carry
all idle connections that sessions don't want to share. They may be
used later to reclaim connections upon socket shortage for example.
diff --git a/src/hlua.c b/src/hlua.c
index 1c2938b..08831ab 100644
--- a/src/hlua.c
+++ b/src/hlua.c
@@ -5060,6 +5060,7 @@
 	socket_tcp.obj_type = OBJ_TYPE_SERVER;
 	LIST_INIT(&socket_tcp.actconns);
 	LIST_INIT(&socket_tcp.pendconns);
+	LIST_INIT(&socket_tcp.priv_conns);
 	socket_tcp.state = SRV_ST_RUNNING; /* early server setup */
 	socket_tcp.last_change = 0;
 	socket_tcp.id = "LUA-TCP-CONN";
@@ -5103,6 +5104,7 @@
 	socket_ssl.obj_type = OBJ_TYPE_SERVER;
 	LIST_INIT(&socket_ssl.actconns);
 	LIST_INIT(&socket_ssl.pendconns);
+	LIST_INIT(&socket_ssl.priv_conns);
 	socket_ssl.state = SRV_ST_RUNNING; /* early server setup */
 	socket_ssl.last_change = 0;
 	socket_ssl.id = "LUA-SSL-CONN";