Always define nested types of a generic as generic as well.
* symtable.pas: 
	+ Add a function which decides whether two defs belong to the same generic.
	* Fix sym_is_owned_by as there can be corner cases where childsym becomes Nil.
* ptype.pas:
	* record_dec: Always define nested types of a generic as generic as well.
	* single_type & read_named_type.expr_type:
		Use the newly added function to improve the checks whether a generic 
		symbol should be rejected as "generics not allowed for types of 
		variables or fields".
+ Add a test for which the improved checks are necessary.

git-svn-id: trunk@20189 -