Change copyObject() to use typeof_unqual

Currently, when the argument of copyObject() is const-qualified, the
return type is also, because the use of typeof carries over all the
qualifiers. This is incorrect, since the point of copyObject() is to
make a copy to mutate. But apparently no code ran into it.

The new implementation uses typeof_unqual, which drops the qualifiers,
making this work correctly.

typeof_unqual is standardized in C23, but all recent versions of all the
usual compilers support it even in non-C23 mode, at least as
__typeof_unqual__. We add a configure/meson test for typeof_unqual and
__typeof_unqual__ and use it if it's available, else we use the existing
fallback of just returning void *.

(Even MSVC supports it, if we use a slightly newer version than is on
CI. For example, the new buildfarm member unicorn would support it.)

The second patch make some changes to take advantage of this improved
qualifier handling. EventTriggerCollectSimpleCommand() is a good
example: It takes a node tree and makes a copy that it keeps around for
its internal purposes, but it can't communicate via its function
signature that it promises not scribble on the passed node tree. That
is now fixed.

The obvious drawback is that if you use an older compiler that supports
typeof but not typeof_unqual, this change will make you lose that check.
Currently, on the Cirrus CI system, only the NetBSD and OpenBSD tasks
are affected by this.

Anyway, the reason I'm posting this now instead of, say, waiting another
year, is that over in the thread "Make copyObject work in C++"[0], we're
discussing, well, making copyObject() work in (standard) C++ (typeof and
typeof_unqual are not in C++). Assuming we want to make the
typeof_unqual change in principle, I figured it would be worth
considering doing that change first and then developing a C++ equivalent
of that, instead of making a C++ equivalent of the current logic and
then having to find another C++ solution when changing to typeof_unqual.

[0]:
https://www.postgresql.org/message-id/flat/CAGECzQR21OnnKiZO_1rLWO0-16kg1JBxnVq-wymYW0-_1cUNtg(at)mail(dot)gmail(dot)com