bpo-38378: Rename parameters "out" and "in" of os.sendfile(). (GH-16742)

They conflicted with keyword "in".

Also rename positional-only parameters of private os._fcopyfile()
for consistency.
diff --git a/Lib/test/test_os.py b/Lib/test/test_os.py
index 4a076e3..d2bd9c2 100644
--- a/Lib/test/test_os.py
+++ b/Lib/test/test_os.py
@@ -3110,11 +3110,12 @@
 
     def test_keywords(self):
         # Keyword arguments should be supported
-        os.sendfile(out=self.sockno, offset=0, count=4096,
-            **{'in': self.fileno})
+        os.sendfile(out_fd=self.sockno, in_fd=self.fileno,
+                    offset=0, count=4096)
         if self.SUPPORT_HEADERS_TRAILERS:
-            os.sendfile(self.sockno, self.fileno, offset=0, count=4096,
-                headers=(), trailers=(), flags=0)
+            os.sendfile(out_fd=self.sockno, in_fd=self.fileno,
+                        offset=0, count=4096,
+                        headers=(), trailers=(), flags=0)
 
     # --- headers / trailers tests