Fix for Mantis #24453. Check for nested types after a specialization. Additionally check correctly whether a type is really a generic before accepting it when parsing a generic.

pgenutil.pas, generate_specialization:
  * use "is_generic" instead of "df_generic in defoptions" as nested non generic types will have that flag set as well and thus would be acceptable for the "<...>" notation although no generic version of it exists
ptype.pas, single_type:
  * check for nested types after doing a specialization

+ added tests (one for now working case and one for now forbidden case)

git-svn-id: trunk@25578 -