Make indent-tabs-mode into a regular mode instead of just a variable

* lisp/simple.el (indent-tabs-mode): Make into a minor mode (bug#6276).

etc/NEWS

@@ -2222,6 +2222,9 @@ This command, called interactively, toggles the local value of
 
 ** Miscellaneous
 
+---
+*** 'indent-tabs-mode' is now a global minor mode instead of just a variable.
+
 ---
 *** New user option 'save-place-abbreviate-file-names'.
 

lisp/simple.el

@@ -6682,6 +6682,10 @@ or \"mark.*active\" at the prompt."
   ;; It's defined in C/cus-start, this stops the d-m-m macro defining it again.
   :variable (default-value 'transient-mark-mode))
 
+(define-minor-mode indent-tabs-mode
+  "Toggle whether indentation can insert TAB characters."
+  :global t :group 'indent :variable indent-tabs-mode)
+
 (defvar widen-automatically t
   "Non-nil means it is ok for commands to call `widen' when they want to.
 Some commands will do this in order to go to positions outside