iOS/tvOS/watchOS/visionOS: Default to kernel-defined backlog in listen

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

Author: madsmtm

std/src/os/unix/net/listener.rs

@@ -81,21 +81,25 @@ impl UnixListener {
             ))]
             const backlog: core::ffi::c_int = 128;
             #[cfg(any(
+                // Silently capped to `/proc/sys/net/core/somaxconn`.
                 target_os = "linux",
+                // Silently capped to `kern.ipc.soacceptqueue`.
                 target_os = "freebsd",
+                // Silently capped to `kern.somaxconn sysctl`.
                 target_os = "openbsd",
-                target_os = "macos"
+                // Silently capped to the default 128.
+                target_vendor = "apple",
             ))]
             const backlog: core::ffi::c_int = -1;
             #[cfg(not(any(
                 target_os = "windows",
                 target_os = "redox",
+                target_os = "espidf",
+                target_os = "horizon",
                 target_os = "linux",
                 target_os = "freebsd",
                 target_os = "openbsd",
-                target_os = "macos",
-                target_os = "espidf",
-                target_os = "horizon"
+                target_vendor = "apple",
             )))]
             const backlog: libc::c_int = libc::SOMAXCONN;