Fix behaviour of the maven update policy

The update policy can be specified using the -U (force update) or -nsu (no update) options, but those options change the whole repository session policy and override any settings on the repositories.

This means that if -U is set, the resolver will attempt to check already downloaded artifacts.  This is wrong and the behaviour has been inherited from maven 2.x.

What we really wants (and what's implied by the name of the options and docs) is to check for new artifacts / updates, so this mainly affect version resolution and not artifact resolution.