src/nsterm.m: fix window tabbing on macOS

* src/nsterm.m: remove NSWindowTabbingModeDisallowed to respect
  system-wide preferences
* etc/NEWS: add mention of native tab support in macOS and where
  to specify system-wide setting

etc/NEWS

@@ -3216,6 +3216,14 @@ The new variable 'xwidget-webkit-download-dir' says where to download to.
 'module-file-suffix' now has the value ".dylib" on macOS, but the
 ".so" suffix is supported as well.
 
++++
+** On macOS, Emacs now supports native tabs (in macOS 10.12 and later).
+Native tabbing behavior is specified system-wide via System
+Preferences, under under General > 'Prefer tabs when opening
+documents'. In macOS versions 10.15 and earlier, this option is
+located under Dock instead. Note that this feature is unrelated to the
+Emacs 'tab-bar-mode'.
+
 +++
 ** On MS-Windows, Emacs can now toggle the IME.
 A new function 'w32-set-ime-open-status' can now be used to disable

src/nsterm.m

@@ -7628,14 +7628,6 @@ - (instancetype) initFrameFromEmacs: (struct frame *)f
          selector:@selector (viewDidResize:)
              name:NSViewFrameDidChangeNotification object:nil];
 
-  /* macOS Sierra automatically enables tabbed windows.  We can't
-     allow this to be enabled until it's available on a Free system.
-     Currently it only happens by accident and is buggy anyway.  */
-#ifdef NS_IMPL_COCOA
-  if ([win respondsToSelector: @selector(setTabbingMode:)])
-    [win setTabbingMode: NSWindowTabbingModeDisallowed];
-#endif
-
   ns_window_num++;
   return self;
 }