Issue 4397: Arrange Protected_scm variables into single list

Issue 4397: Arrange Protected_scm variables into single list

Since we need to allocate one cons cell per protected variable anyway,
we can just arrange them into a single list, making it possible to

a) just require a single scm_permanent_object call, namely for the head

b) have a representation able to return all protected elements, should
we require this at some later point of time