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: http://svn.freepascal.org/svn/fpc/trunk@25578 3ad0048d-3df7-0310-abae-a5850022a9f2