betterC and core.stdc modules; betterC and varargs routines

[ryuukk_]

```D

import core.stdc.stdarg;


// add this
extern(C) void va_end(void* ap){}

pragma(printf)
public extern(C) int wrap_printf(scope const char* f, ...) {
     import core.stdc.stdio;

     va_list ap;

     va_start(ap, f);
     auto rc = vprintf(f, ap);
     va_end(ap);

     return rc;
}


extern(C) void main(int argc, const(char)** argv)
{
     wrap_printf("Foo\n");
}

```

Adding: `extern(C) void va_end(void* ap){}` works for me, looks 
like DMD doesn't do its job properly

If -betterC can't do what C does, then it's not -betterC, but 
-worseC

To me this is a bug and it should be reported