This is a step towards converting `wopen_cloexec()` to return `File` instead of
`OwnedFd`/`AutocloseFd`.¹
In addition to letting us use native standard library functions instead of
unsafe libc calls, we gain additional semantic safety because `File` operations
that manipulate the state of the fd (e.g. `File::seek()`) require a `&mut`
reference to the `File`, whereas using `RawFd` or `OwnedFd` everywhere leaves us
in a position where it's not clear whether or not other references to the same
fd will manipulate its underlying state.
¹ We actually wouldn't even need `wopen_cloexec()` at all (just a widechar
wrapper) as Rust's native `File::open()`/`File::create()` functionality uses
`FD_CLOEXEC` internally.
I was able to trigger this by flipping around the history pager.
Since the only applicable caller here already stops if it gets None,
just don't assert.
We still don't support tabs but as of the parent commit, there are no more
weird glitches, so it should be fine to recall those lines?
This reverts commit cc0e366037.
The lines of code I commented on in #10254 were meant to serve only as examples
of the changes I was requesting, not the only instances.
Also just use `Mode::from_bits_truncate()` instead of unsafe or unwrapping since
we know the modes are correct.
