service: Rename secret_service_new() and friends to open()

The _new() suffix confuses vala and gobject introspection thinking
that it's a constructor, and there's no way to tell it otherwise. And
things really get messy because they're async functions.

So while we're still unstable, rename these functions to
secret_service_open() secret_service_open_sync() and
secret_service_open_finish().

This is an API/API break, but its to the portion of the library
marked as unstable. Only used by seahorse (in jhbuild) and updated
usage there.