…nfly.

This behavior is defined in general for the XNU kernel, not just macOS:
https://github.com/apple-oss-distributions/xnu/blob/rel/xnu-10002/bsd/kern/uipc_socket.c

Co-authored-by: Jubilee <46493976+workingjubilee@users.noreply.github.com>

Tested in the iOS simulator that the thread name is not set by default,
and that setting it improves the debugging experience in lldb / Xcode.

Tested on an old MacBook and the iOS simulator.

Tested in the iOS simulator with something like:
```
let mut buf = vec![0; c_int::MAX as usize - 1 + 2];
let read_bytes = f.read(&mut buf).unwrap();
```

This uses `libc::fcntl`, which, while not explicitly marked as available
in the headers, is already used by `File::sync_all` and `File::sync_data`
on these platforms, so should be fine to use here as well.

…gjubilee

std::net: Socket::new_raw now set to SO_NOSIGPIPE on freebsd.

…exists, r=workingjubilee

add note about `AlreadyExists` to `create_new`

Fixes rust-lang#119244

…workingjubilee

Convert instances of `target_os = "macos"` to `target_vendor = "apple"`

rust-lang#124491 migrated towards using `target_vendor = "apple"` more, as there's very little difference between iOS, tvOS, watchOS and visionOS. In that PR, I only did the changes where the standard library already had fixes for iOS, that I could confidently apply to the other targets.

However, there's actually also not that big of a gap between macOS and the aforementioned platforms - so in this PR, I've gone through a few of the instances of `target_os = "macos"` and replaced it with `target_vendor = "apple"` to improve support on those platforms, see the commits for details.

r? workingjubilee

CC `@thomcc` `@simlay` (do tell me if I should stop pinging you on these Apple PRs)

`@rustbot` label O-apple

…tmcm

next_power_of_two: add a doctest to show what happens on 0