If user re-DECLAREs an existing cursor name, close the old cursor of

If user re-DECLAREs an existing cursor name, close the old cursor of
that name and issue a NOTICE to the effect that we did.  Previously,
code would try to assign the new cursor declaration to the old portal,
but this didn't work reliably since new parsetree is still sitting in
blank portal and is likely to get clobbered.