Support fd_fdstat_get and fd_renumber on stdin/stdout/stderr (#631)

* Support fd_fdstat_get on stdin/stdout/stderr.

Add a routine for obtaining an `OsFile` containing a file descriptor for
stdin/stdout/stderr so that we can do fd_fdstat_get on them.

* Add a testcase for fd_fdstat_get etc. on stdin etc.

* Don't dup file descriptors in fd_renumber.

* Fix compilation on macOS

* Rename OsFile to OsHandle

This commits renames `OsFile` to `OsHandle` which seems to make
more sense semantically as it is permitted to hold a valid OS handle
to OS entities other than simply file/dir (e.g., socket, stream, etc.).
As such, this commit also renames methods on `Descriptor` struct
from `as_actual_file` to `as_file` as this in reality does pertain
ops on FS entities such as files/dirs, and `as_file` to `as_os_handle`
as in this case it can be anything, from file, through a socket, to
a stream.

* Fix compilation on Linux

* Introduce `OsHandleRef` for borrowing OS resources.

To prevent a `ManuallyDrop<OsHandleRef>` from outliving the resource it
holds on to, create an `OsHandleRef` class parameterized on the lifetime
of the `Descriptor`.

* Fix scoping to pub-priv and backport to snapshot_0

crates/wasi-common/src/sys/unix/bsd/oshandle.rs

@@ -0,0 +1,48 @@
+use super::super::dir::Dir;
+use std::fs;
+use std::ops::{Deref, DerefMut};
+use std::os::unix::prelude::{AsRawFd, RawFd};
+use std::sync::Mutex;
+
+#[derive(Debug)]
+pub(crate) struct OsHandle {
+    pub(crate) file: fs::File,
+    // In case that this `OsHandle` actually refers to a directory,
+    // when the client makes a `fd_readdir` syscall on this descriptor,
+    // we will need to cache the `libc::DIR` pointer manually in order
+    // to be able to seek on it later. While on Linux, this is handled
+    // by the OS, BSD Unixes require the client to do this caching.
+    //
+    // This comes directly from the BSD man pages on `readdir`:
+    //   > Values returned by telldir() are good only for the lifetime
+    //   > of the DIR pointer, dirp, from which they are derived.
+    //   > If the directory is closed and then reopened, prior values
+    //   > returned by telldir() will no longer be valid.
+    pub(crate) dir: Option<Mutex<Dir>>,
+}
+
+impl From<fs::File> for OsHandle {
+    fn from(file: fs::File) -> Self {
+        Self { file, dir: None }
+    }
+}
+
+impl AsRawFd for OsHandle {
+    fn as_raw_fd(&self) -> RawFd {
+        self.file.as_raw_fd()
+    }
+}
+
+impl Deref for OsHandle {
+    type Target = fs::File;
+
+    fn deref(&self) -> &Self::Target {
+        &self.file
+    }
+}
+
+impl DerefMut for OsHandle {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.file
+    }
+}