libstd: io::fs::File::stat() need not to take &mut self.

Signed-off-by: NODA, Kai <nodakai@gmail.com>
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>
auto merge of #19303 : nodakai/rust/libsyntax-reject-dirs, r=alexcrichton

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`.

Incidentally, remove unnecessary mutability of `&self` from `io::fs::File::stat()`.