Dragonfly: expose __errno_location() rather than define __error().

Expose __errno_location() (introduced in DragonFlyBSD 5.8), which
returns the current thread's errno value. This is similar to Linux
and avoids having a separate module that defines both errno (which
depends on the thread_local feature) and an __error() function.

Author: cmusser

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

@@ -1044,6 +1044,7 @@ f! {
 }
 
 extern "C" {
+    pub fn __errno_location() -> *mut ::c_int;
     pub fn setgrent();
     pub fn mprotect(
         addr: *mut ::c_void,
@@ -1077,10 +1078,3 @@ extern "C" {
         needlelen: ::size_t,
     ) -> *mut ::c_void;
 }
-
-cfg_if! {
-    if #[cfg(libc_thread_local)] {
-        mod errno;
-        pub use self::errno::*;
-    }
-}