* 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
1f9d764d5d