Reworked locked narrowing.

Reworked locked narrowing.

* src/editfns.c: (narrowing_locks): New alist to hold the narrowing
locks and their buffers.
(narrowing_lock_get_bound, narrowing_lock_peek_tag)
(narrowing_lock_push, narrowing_lock_pop): New functions to access
and update 'narrowing_locks'.
(reset_outermost_narrowings, unwind_reset_outermost_narrowing):
Functions moved from src/xdisp.c, and rewritten with the above
functions.
(Fwiden): Use the above functions. Update docstring.
(Fnarrow_to_region, Fnarrowing_lock, Fnarrowing_unlock): Use the above
functions.
(syms_of_editfns): Remove the 'narrowing-locks' variable.

* src/lisp.h: Make 'reset_outermost_narrowings' externally visible.

* src/xdisp.c (reset_outermost_narrowings)
unwind_reset_outermost_narrowing): Functions moved to src/editfns.c.

* lisp/subr.el (with-locked-narrowing): Improved macro, with a helper
function.