MEDIUM: protocol: use a custom AF_MAX to help protocol parser

It's possible to have several protocols per family which is a problem
with the current way the protocols are stored.

This allows to register a new protocol in HAProxy which is not a
protocol in the strict socket definition. It will be used to register a
SOCK_STREAM protocol using socketpair().
diff --git a/include/proto/protocol.h b/include/proto/protocol.h
index 13a3c0a..857f9e6 100644
--- a/include/proto/protocol.h
+++ b/include/proto/protocol.h
@@ -25,7 +25,7 @@
 #include <sys/socket.h>
 #include <types/protocol.h>
 
-extern struct protocol *__protocol_by_family[AF_MAX];
+extern struct protocol *__protocol_by_family[AF_CUST_MAX];
 
 /* Registers the protocol <proto> */
 void protocol_register(struct protocol *proto);
@@ -56,7 +56,7 @@
 /* returns the protocol associated to family <family> or NULL if not found */
 static inline struct protocol *protocol_by_family(int family)
 {
-	if (family >= 0 && family < AF_MAX)
+	if (family >= 0 && family < AF_CUST_MAX)
 		return __protocol_by_family[family];
 	return NULL;
 }