Coerce unknown-literal-constant default values to the column type during

Coerce unknown-literal-constant default values to the column type during
CREATE TABLE (or ALTER TABLE SET DEFAULT), rather than postponing it to
the time that the default is inserted into an INSERT command by the
rewriter.  This reverses an old decision that was intended to make the
world safe for writing
	f1 timestamp default 'now'
but in fact merely made the failure modes subtle rather than obvious.
Per recent trouble report and followup discussion.

initdb forced since there is a chance that stored default expressions
will change.