More work in prep for having wopen_cloexec() return `File` directly.
This eliminates checking for an invalid fd and makes both ownership and
mutability clear (some more operations that involve changes to the underlying
state of the fd now require `&mut File` instead of just a `RawFd`).
Code that clearly does not use non-blocking IO is ported to use
`Write::write_all()` directly instead of our rusty port of the `write_loop()`
function (which handles EAGAIN/EWOULDBLOCK in addition to EINTR, while
`write_all()` only handles the latter).
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.
