fix default value of Product.targetName

Do not use rfc1034Identifier for the default value of
Product.targetName. Instead replace invalid file name characters with
underscores. This amends 81500ce1.

Task-number: QBS-637
Change-Id: Id341e3092b9b3efc7d4a76ee3b8871383e6d4154
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>