improving C++ libc headers

hi list,

i am considering working up a patch to improve C++ libc headers in FreeBSD.

the problem with the current headers (which come from clang/libc++) is
that this code compiles:

------
#include <cstdlib>
auto main() -> int {
	exit(0);
}
------

this should be a compile-time error, because <cstdio> does not declare
::exit[0].  however, it works because <cstdlib> is implemented like
this:

------
#include <stdlib.h> // exposes ::exit()

namespace std {
	using ::exit;
	// ...
}
------

i would like to replace this with an implementation that does this
instead:

------
namespace std {
	extern "C" void exit(int);
	// ...
}
------

i have done a very quick proof of concept for this and it does work;
there are some more complicated edge cases, but nothing that can't be
dealt with from what i can see.

but before i put any amount of significant effort into this, i'd like to
check if this is something that is likely to be merged.

to address some potential objections:

- i don't believe this can be handled upstream in libc++, because some
  of these headers require knowledge of the implementation - <cstdio>
  needs to know about 'struct __sFILE', for example.

- "import std" should solve this properly, but there are a lot of
  implementation issues with this and it's not clear if/when it will
  ever be supported; in the mean time, the existing standard headers
  should behave correctly.

[0] https://www.eel.is/c++draft/cstdlib.syn#header:%3Ccstdlib%3E
[1] https://cgit.freebsd.org/src/tree/contrib/llvm-project/libcxx/include/cstdlib