Improved locked narrowing.

Improved locked narrowing.

* src/editfns.c (Fnarrowing_lock, Fnarrowing_unlock,
narrow_to_region_locked, unwind_narrow_to_region_locked):
New functions.
(Fnarrow_to_region, Fwiden): Adapt, and make it possible to use
these functions within the bounds of the locked narrowing.
(syms_of_editfns): Change the name of the variable, make it
buffer-local, and add the two Snarrowing_lock and Snarrowing_unlock
subroutines.

* src/lisp.h: Prototype of 'narrow_to_region_locked'.

* src/xdisp.c (handle_fontified_prop):
* src/keyboard.c (safe_run_hooks_maybe_narrowed): Use
'narrow_to_region_locked'.

* lisp/subr.el (with-locked-narrowing): New macro.