hubic: fix uploads - fixes #2524

Uploads were broken because chunk size was set to zero.  This was a
consequence of the backend config re-organization which meant that
chunk size had lost its default.

Sharing some backend config between swift and hubic fixes the problem
and means hubic gains its own --hubic-chunk-size flag.

backend/hubic/hubic.go

@@ -60,13 +60,13 @@ func init() {
 				log.Fatalf("Failed to configure token: %v", err)
 			}
 		},
-		Options: []fs.Option{{
+		Options: append([]fs.Option{{
 			Name: config.ConfigClientID,
 			Help: "Hubic Client Id\nLeave blank normally.",
 		}, {
 			Name: config.ConfigClientSecret,
 			Help: "Hubic Client Secret\nLeave blank normally.",
-		}},
+		}}, swift.SharedOptions...),
 	})
 }