Fix and improve release bumping algorithm
    
    Previously, `evr` was overloaded, meaning both "latest existing version"
    (mainly for testing) and "epoch-version for which we want a release". If
    a new version was specified, this caused the latest existing release to
    be assumed as "1", giving "2" as a result. This change separates these
    two concerns.
    
    Additionally, if nothing is specified the code looks into the spec file
    for the [epoch:]version we need to work with.
    
    Also, name some things more uniformly, e.g. use "next" consistently
    instead of "new".
    
    Signed-off-by: Nils Philippsen <nils@redhat.com>