Introduce a decision metric (dmetric) that classifies the relation of

Introduce a decision metric (dmetric) that classifies the relation of
this prefix with respect to its previous one.

Currently the plan is to distinguish the best prefix (only one), ecmp
prefixes (currently the same as as-wide-multipath), as-wide-multipath
prefixes, valid prefixes and invalid prefixes.
This information will be used to implement add-path send but also for
ecmp support in bgpd.
OK tb@