PL/pgSQL cursors should get generated portal names by default

There's a complaint at [1] about how you can't re-use the same
cursor variable name within a routine called from another routine
that's already using that name. The complaint is itself a bit
under-documented, but I believe it is referring to this ancient
bit of behavior:

A bound cursor variable is initialized to the string value
representing its name, so that the portal name is the same as
the cursor variable name, unless the programmer overrides it
by assignment before opening the cursor.

So if you try to nest usage of two bound cursor variables of the
same name, it blows up on the portal-name conflict. But it'll work
fine if you use unbound cursors (i.e., plain "refcursor" variables):

But an unbound cursor
variable defaults to the null value initially, so it will receive
an automatically-generated unique name, unless overridden.

I wonder why we did it like that; maybe it's to be bug-compatible with
some Oracle PL/SQL behavior or other? Anyway, this seems non-orthogonal
and contrary to all principles of structured programming. We don't even
offer an example of the sort of usage that would benefit from it, ie
that calling code could "just know" what the portal name is.

I propose that we should drop this auto initialization and let all
refcursor variables start out null, so that they'll get unique
portal names unless you take explicit steps to do something else.
As attached.

(Obviously this would be a HEAD-only fix, but maybe there's scope for
improving the back-branch docs along lines similar to these changes.)

regards, tom lane

[1] https://www.postgresql.org/message-id/166689990972.627.16269382598283029015%40wrigleys.postgresql.org