Further fixes for bogus list-slinging, scribbling on input, etc in type

Further fixes for bogus list-slinging, scribbling on input, etc in type
coercion code.  I'm beginning to wonder why we have separate candidate
selection routines for functions, operators, and aggregates --- shouldn't
this code all be unified?  But meanwhile,
	SELECT 'a' LIKE 'a';
finally works; the code for dealing with unknown input types for operators
was pretty busted.