Windows' `NtCreateFile` has an ability to take a directory and a
relative path, so use that to implement `open_unchecked` instead of
using path concatenation.

We still use concatenation for other functions, but this is the first
step to rewriting those to avoid it.

Fixes #226.

Always use `open_unchecked` with no access for implementing `stat` on Windows,
as that now uses the new `NtCreateFile` path.

Use `let _ = ambient_authority;` to suppress unused argument warnings
rather than naming arguments `_`, because argument names show up in the
documentation, and the fact that these arguments are unused is not part
of the public interface.

* Add a `Pool::insert_ip_net_any_port` function.

Add a function to `Pool` to allow inserting a network that can accept
any port.

* Add support for port ranges, and add some unit tests.

Similar to dtolnay/anyhow#248, and as suggested [here], check `RUSTC_WRAPPER` instead of `CARGO_RUSTC_WRAPPER`.

[here]: bytecodealliance/rustix#544 (comment)

Port bytecodealliance/rustix#565 to cap-std.

`File::create_ambient` is to `File::open_ambient` as `std::fs::File::create` is
to `std::fs::File::open`.

Ubuntu 18.04 is no longer supported on Github actions.

The only code change here is due to `copy_file_range`'s `len` argument
changing from `u64` to `usize`.