Restore the __error() function, but add a deprecation warning.

Author: cmusser

src/unix/bsd/freebsdlike/dragonfly/errno.rs

@@ -0,0 +1,13 @@
+// DragonFlyBSD's __error function is declared with "static inline", so it must
+// be implemented in the libc crate, as a pointer to a static thread_local.
+f! {
+    #[deprecated(since = "0.2.77", "Use `__errno_location()` instead")]
+    pub fn __error() -> *mut ::c_int {
+        &mut errno
+    }
+}
+
+extern "C" {
+    #[thread_local]
+    pub static mut errno: ::c_int;
+}

src/unix/bsd/freebsdlike/dragonfly/mod.rs

@@ -1078,3 +1078,10 @@ extern "C" {
         needlelen: ::size_t,
     ) -> *mut ::c_void;
 }
+
+cfg_if! {
+    if #[cfg(libc_thread_local)] {
+        mod errno;
+        pub use self::errno::*;
+    }
+}