[MEDIUM] Implement and use generic findproxy and relax duplicated proxy check

This patch:
 - adds proxy_mode_str() similar to proxy_type_str()
 - adds a generic findproxy function used with default_backend/setbe/use_backed
 - rewrite default_backend/senbe/use_backed to use introduced findproxy()
 - relaxes duplicated proxy check
 - changes capabilities displaying from "%X" to "%s" with a call to proxy_type_str()
diff --git a/include/proto/proxy.h b/include/proto/proxy.h
index 38d7325..839cce5 100644
--- a/include/proto/proxy.h
+++ b/include/proto/proxy.h
@@ -32,8 +32,10 @@
 void pause_proxy(struct proxy *p);
 void pause_proxies(void);
 void listen_proxies(void);
-const char *proxy_type_str(struct proxy *proxy);
 
+const char *proxy_type_str(struct proxy *proxy);
+const char *proxy_mode_str(int mode);
+struct proxy *findproxy(const char *name, int mode, int cap);
 
 #endif /* _PROTO_PROXY_H */