an error if the last component of the path does not exist.
This is a bug in libgnat. The System.File_IO.Open function calls
full_name -> __gnat_full_name -> realpath on files that have not
yet been created. The Linux realpath was considered lacking, so a
different approach is used there. We now remove OpenBSD from the
#ifdef of operating systems supporting realpath and follow the Linux
code path.
New Ada bootstraps are required.
ok sthen@ pascal@