[OPTIM] introduce global parameter "tune.maxaccept"

This new parameter makes it possible to override the default
number of consecutive incoming connections which can be
accepted on a socket. By default it is not limited on single
process mode, and limited to 8 in multi-process mode.
diff --git a/src/client.c b/src/client.c
index 2505db3..cfcfb0c 100644
--- a/src/client.c
+++ b/src/client.c
@@ -74,12 +74,7 @@
 	struct http_txn *txn;
 	struct task *t;
 	int cfd;
-	int max_accept;
-
-	if (global.nbproc > 1)
-		max_accept = 8; /* let other processes catch some connections too */
-	else
-		max_accept = -1;
+	int max_accept = global.tune.maxaccept;
 
 	while (p->feconn < p->maxconn && max_accept--) {
 		struct sockaddr_storage addr;