Rationalize calendar handling of day and month abbrev-arrays.

Rationalize calendar handling of day and month abbrev-arrays.

* lisp/calendar/calendar.el (calendar-customized-p): New function.
(calendar-abbrev-construct, calendar-make-alist): Change what it does.
(calendar-day-name-array, calendar-month-name-array): Doc fix.
Add :set function.
(calendar-abbrev-length, calendar-day-abbrev-array)
(calendar-month-abbrev-array): Make defcustoms, with appropriate :set.
(calendar-day-abbrev-array, calendar-month-abbrev-array):
Elements may no longer be nil.
(calendar-day-name, calendar-month-name):
Update for changed nature of abbrev arrays.
* calendar/diary-lib.el (diary-name-pattern):
Update for changed nature of abbrev arrays.
(diary-mark-entries-1): Update calendar-make-alist calls.
(diary-font-lock-date-forms): Doc fix for changed abbrev arrays.
* calendar/cal-html.el (cal-html-day-abbrev-array):
Simply inherit from calendar-day-abbrev-array.

* etc/NEWS: Mention this.