Enable explicitly setting mode property to manual

Use update-alternatives --set to change the value of the link to the current
value to ensure the alternative is set in "manual" mode.

This fixes a couple of issues:
  - setting a path and mode => 'manual' would previously cause the mode to
    change to manual when the path was changed, but mode=(:manual) would be
    called which blindly called update-alternatives --auto
  - allows the mode to be forced into manual without actively changing the
    value of the link, preventing it being automatically changed later
Merge pull request #17 from domcleal/mode

Enable explicitly setting mode property to manual