libc: Fix getentropy POSIX 2024 conformance issues

[emaste]

Summary

GETENTROPY_MAX should be defined in limits.h.  EINVAL is the return
value for buflen > GETENTROPY_MAX.

[emaste]

I'm not sure how the build is intended to set __POSIX_VISIBLE for building libc's getentropy.c.

[imp]

include/limits.h

142

Why doesn't this work today? I've recently bumped __POSIX_VISIBLE to 202405 for the default BSD_VISIBLE sources

[imp]

In D47689#1087553, @emaste wrote:

I'm not sure how the build is intended to set __POSIX_VISIBLE for building libc's getentropy.c.

By default we build with BSD_SOURCE which makes it all visible.

[emaste]

Oh, on ref15-amd64.freebsd.org I get:

#include <sys/cdefs.h>
#include <stdio.h>

int
main(int argc, char *argv[])
{
        printf("__POSIX_VISIBLE=%d\n", __POSIX_VISIBLE);
}

__POSIX_VISIBLE=200809

[emaste]

Restore POSIX_VISIBLE tests - issue was local cross-threading