Procedure additions to ParseFuncOrColumn are inadequate

I noticed that ParseFuncOrColumn isn't terribly thorough about rejecting
non-procedure results when proc_call is true.  Since the caller is just
assuming that it gets back what it expects, that leads to core dumps
or worse:

regression=# call int4(42);
server closed the connection unexpectedly
        This probably means the server terminated abnormally

Also, with the existing ad-hoc approach to this, not all the ereports
are alike; the one for an aggregate lacks the HINT the others provide.

Also, in the cases where it does successfully reject should-be-procedure
or should-not-be-procedure, it reports ERRCODE_UNDEFINED_FUNCTION, which
seems quite inappropriate from here: I'd expect ERRCODE_WRONG_OBJECT_TYPE.

There's also a pre-existing oversight that when we get a result of
FUNCDETAIL_COERCION, we should reject that if there's any aggregate
decoration, but we fail to.

The attached cleans all this up; any objections?

			regards, tom lane