[MINOR] frontend: add a new analyser to parse a proxied connection

The introduction of a new PROXY protocol for proxied connections requires
an early analyser to decode the incoming connection and set the session
flags accordingly.

Some more work is needed, among which setting a flag on the session to
indicate it's proxied, and copying the original parameters for later
comparisons with new ACLs (eg: real_src, ...).
diff --git a/include/proto/frontend.h b/include/proto/frontend.h
index 796ebbb..2f0930f 100644
--- a/include/proto/frontend.h
+++ b/include/proto/frontend.h
@@ -27,6 +27,7 @@
 
 void get_frt_addr(struct session *s);
 int frontend_accept(struct session *s);
+int frontend_decode_proxy_request(struct session *s, struct buffer *req, int an_bit);
 
 
 #endif /* _PROTO_FRONTEND_H */