It also update the route's id.

The field "result" is in fact an additional cost.

This is a MUST according to the Babel Extension Mechanism draft
(Section 2.2.3).

This reverts commit bf8555b2.

This is consistent with Baptiste's work, and is better in any case.

Makes it consistent with the other interface flags.

Without this, later interface definitions override earlier ones
even when they don't specify a value.