Fix func_rs() and as_subselect_rs() to start behaving as advertised

Fix func_rs() and as_subselect_rs() to start behaving as advertised

No idea how it never got noticed, but both have been broken since the very
first commits that introduced the methods ( 4fa7bc22 / e4bb6727 ). While
changing them 7 years later is a rather serious modification of behavior,
the old way never worked without users having to force-scalar each call site.

If someone has been relying on e.g. [ func_rs(...) ] to return actual result
objects instead of the resultset instance - things will blow up rather quickly
and loudly (aside from the carp()-ed warning encouraging users to switch to
scalar ctx explicitly)

[ func( ... ) ] of course continues to behave like before (directly returning
raw values off the cursor... sigh)