unix: set errno in uv_fs_copyfile()

When the specific value of -1 is returned, uv__fs_work() uses
the value of errno. This commit sets errno in uv__fs_copyfile().

Fixes: nodejs/node#21329
PR-URL: libuv#1881
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

src/unix/fs.c

@@ -929,7 +929,11 @@ static ssize_t uv__fs_copyfile(uv_fs_t* req) {
     }
   }
 
-  return result;
+  if (result == 0)
+    return 0;
+
+  errno = UV__ERR(result);
+  return -1;
 #endif
 }