Client: assume uid==0 is local socket

A negative uid value means it's not a "local socket" (PF_LOCAL).
uid==0 means user "root" connected.

src/Client.hxx

@@ -109,7 +109,7 @@ public:
 	 * a local (UNIX domain) socket?
 	 */
 	bool IsLocal() const {
-		return uid > 0;
+		return uid >= 0;
 	}
 
 	unsigned GetPermission() const {

src/ClientFile.cxx

@@ -47,7 +47,7 @@ client_allow_file(const Client &client, Path path_fs, Error &error)
 		   instance */
 		return true;
 
-	if (uid <= 0) {
+	if (uid < 0) {
 		/* unauthenticated client */
 		error.Set(ack_domain, ACK_ERROR_PERMISSION, "Access denied");
 		return false;