Add an optional 'lock' parameter to 'narrow-to-region'

* src/editfns.c (Fnarrow_to_region): Add the parameter to the
function, and handle it. Update docstring.
(unwind_locked_begv, unwind_locked_zv): New functions.
(Fwiden): Do nothing when restrictions are locked. Update
docstring.
(syms_of_editfns): Replace the 'inhibit-widen' symbol and variable
with a 'restrictions-locked' symbol and variable. Update docstring.

* src/xdisp.c (handle_fontified_prop): Use Fnarrow_to_region with
the new parameter.
(unwind_narrowed_zv): Remove function.

* src/process.c (Finternal_default_process_filter): Add a third
argument to Fnarrow_to_region.

* src/lread.c (readevalloop): Add a third argument to
Fnarrow_to_region.

* src/bytecode.c (exec_byte_code): Add a third argument to
Fnarrow_to_region.

* etc/NEWS (like): Mention the new parameter of 'narrow-to-region'.

* doc/lispref/positions.texi (Narrowing): Document it.

src/bytecode.c

@@ -1481,7 +1481,7 @@ exec_byte_code (Lisp_Object fun, ptrdiff_t args_template,
 	CASE (Bnarrow_to_region):
 	  {
 	    Lisp_Object v1 = POP;
-	    TOP = Fnarrow_to_region (TOP, v1);
+	    TOP = Fnarrow_to_region (TOP, v1, Qnil);
 	    NEXT;
 	  }