libstd: explicitly disallow io::fs::File to open a directory.

On *BSD systems, we can open(2) a directory and directly read(2) from
it due to an old tradition.  We should avoid doing so by explicitly
calling fstat(2) to check the type of the opened file.

Opening a directory as a module file can't always be avoided.
Even when there's no "path" attribute trick involved, there can always
be a *directory* named "my_module.rs".

Fix #12460

Signed-off-by: NODA, Kai <nodakai@gmail.com>

src/test/compile-fail/issue-5806.rs

@@ -18,9 +18,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// ignore-freebsd FIXME #12460
-
 #[path = "../compile-fail"]
-mod foo; //~ ERROR: illegal operation on a directory
+mod foo; //~ ERROR: a directory
 
 fn main() {}