Re: Identifying function-lookup failures due to argument name mismatches

Here is a v4 with some additional bike-shedding on the error texts.
In particular, I decided that it was worth expending an additional
flag bit so that we could reliably distinguish "There is no function
of that name" from "A function of that name exists, but it is not in
the search_path".  (Since FuncnameGetCandidates is already searching
the entire set of functions matching the given name, it doesn't take
any extra work to know that there's a match outside the search path.)
I rephrased a couple of the other messages too, but without any
substantive logic change.

			regards, tom lane