cmake: set.cmake: fix qi_persistent_{pre,ap}pend_uniq functions

Because qi_persistent_prepend_uniq and qi_perssistent_append_uniq are
functions, they must store their result in their parent scope;
otherwise, the result is lost as soon as the function is exited.
cmake: set.cmake: cleanup deprecation messages

Do not recommend usage of non-existing functions or macros in the
deprecation messages.