Auto merge of #85746 - m-ou-se:io-error-other, r=joshtriplett

Redefine `ErrorKind::Other` and stop using it in std.

This implements the idea I shared yesterday in the libs meeting when we were discussing how to handle adding new `ErrorKind`s to the standard library: This redefines `Other` to be for *user defined errors only*, and changes all uses of `Other` in the standard library to a `#[doc(hidden)]` and permanently `#[unstable]` `ErrorKind` that users can not match on. This ensures that adding `ErrorKind`s at a later point in time is not a breaking change, since the user couldn't match on these errors anyway. This way, we use the `#[non_exhaustive]` property of the enum in a more effective way.

Open questions:
- How do we check this change doesn't cause too much breakage? Will a crate run help and be enough?
- How do we ensure we don't accidentally start using `Other` again in the standard library? We don't have a `pub(not crate)` or `#[deprecated(in this crate only)]`.

cc https://github.com/rust-lang/rust/pull/79965

cc `@rust-lang/libs` `@ijackson`

r? `@dtolnay`

library/std/src/sys/unix/net.rs

@@ -38,7 +38,7 @@ pub fn cvt_gai(err: c_int) -> io::Result<()> {
         str::from_utf8(CStr::from_ptr(libc::gai_strerror(err)).to_bytes()).unwrap().to_owned()
     };
     Err(io::Error::new(
-        io::ErrorKind::Other,
+        io::ErrorKind::Uncategorized,
         &format!("failed to lookup address information: {}", detail)[..],
     ))
 }
@@ -178,7 +178,7 @@ impl Socket {
                     if pollfd.revents & libc::POLLHUP != 0 {
                         let e = self.take_error()?.unwrap_or_else(|| {
                             io::Error::new_const(
-                                io::ErrorKind::Other,
+                                io::ErrorKind::Uncategorized,
                                 &"no error set after POLLHUP",
                             )
                         });