add checks for items and bionics of professions
recipes: check for book-to-learn-from exists and is a book else print a debug message
handle re-definitions of vehicle parts gracefully
handle unknown part types in vehicle prototypes
check spawn definitions in vehicle protoypes

added Item_factory::has_group to check if an item group exists
Check professions gender specific starting items, too.
Remove comment.

Addictions are already checked when creating an instance of addiction in profession::load_profession
Check starting skills in profession.
Make map::spawn_item ignore item ids that would lead to a null item on the map.
remove the GUITAR spawn from fire trucks.

The item currently does not exists, can be added later when #1980 gets fixed.