Re: Improving GUC prefix ownership for extensions

> What MarkGUCPrefixReserved is really about is detecting
> misspelled hand-made config-file entries and SET commands as best we
> can.

That's not strictly true, MarkGUCPrefixReserved already implements part of this:

1. If extension A reserves prefix "ext_a"
2. And then extension B tries to register a GUC named "ext_a.something"

We get an error.

But if these extensions are loaded in the opposite order, everything
works fine, extension B can use a variable in a prefix reserved by
another extension.

If I remove the part about requiring MarkGUCPrefixReserved prefix /
printing a warning if it's not called, all this patch does is makes
the behavior of MarkGUCPrefixReserved more consistent.

My initial goal was simply that (to print out an error even if things
happen in a different order), but then I thought maybe also nudging
extensions towards reserving prefixes with MarkGUCPrefixReserved would
be useful.

> I'm also skeptical that there's any
> real-world problem that needs solving here

This came up based on a discussion on how MarkGUCPrefixReserved works,
and what exactly does it prevent, because it's not really
consistent/intuitive currently. I personally thought that it already
enforces that only that extension can use the prefix, and I was
surprised when I discovered that it is only half true.

What do you think about a simplified version, which only solves the
original problem - properly reporting errors regardless of the order
of library loading, if MarkGUCPrefixReserved is used?