EXECUTE of a SELECT ... INTO now draws a 'not implemented' error,

EXECUTE of a SELECT ... INTO now draws a 'not implemented' error,
rather than executing the INTO clause with non-plpgsql semantics
as it was doing for the last few weeks/months.  This keeps our options
open for making it do the right plpgsql-ish thing in future without
creating a backwards compatibility problem.  There is no loss of
functionality since people can get the same behavior with CREATE TABLE AS.