std: fix unintended cross-target changes in QuRT support

- Gate restricted_std behind cfg_attr(target_os = "qurt") in test crate
- Restore VxWorks-specific error messages for lchown/chroot
- Remove spurious reason field from peer_credentials_unix_socket unstable attr

Author: androm3da

library/std/src/os/unix/net/mod.rs

@@ -55,5 +55,5 @@ pub use self::stream::*;
     target_vendor = "apple",
     target_os = "cygwin",
 ))]
-#[unstable(feature = "peer_credentials_unix_socket", issue = "42839", reason = "unstable")]
+#[unstable(feature = "peer_credentials_unix_socket", issue = "42839")]
 pub use self::ucred::*;

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

@@ -2563,7 +2563,7 @@ pub fn lchown(path: &Path, uid: u32, gid: u32) -> io::Result<()> {
 
 #[cfg(target_os = "vxworks")]
 pub fn lchown(_path: &Path, _uid: u32, _gid: u32) -> io::Result<()> {
-    Err(io::const_error!(io::ErrorKind::Unsupported, "lchown not supported"))
+    Err(io::const_error!(io::ErrorKind::Unsupported, "lchown not supported by vxworks"))
 }
 
 #[cfg(not(any(
@@ -2578,7 +2578,7 @@ pub fn chroot(dir: &Path) -> io::Result<()> {
 
 #[cfg(target_os = "vxworks")]
 pub fn chroot(_dir: &Path) -> io::Result<()> {
-    Err(io::const_error!(io::ErrorKind::Unsupported, "chroot not supported"))
+    Err(io::const_error!(io::ErrorKind::Unsupported, "chroot not supported by vxworks"))
 }
 
 #[cfg(not(any(target_os = "wasi", target_os = "qurt")))]

library/test/src/lib.rs

@@ -26,7 +26,7 @@
 #![feature(panic_can_unwind)]
 #![cfg_attr(test, feature(test))]
 #![feature(thread_spawn_hook)]
-#![feature(restricted_std)]
+#![cfg_attr(target_os = "qurt", feature(restricted_std))]
 #![allow(internal_features)]
 #![warn(rustdoc::unescaped_backticks)]
 #![warn(unreachable_pub)]