Add new minor mode `header-line-indent-mode'

Add new minor mode `header-line-indent-mode'

This is mostly factored out from tabulated-list.el (with bugs fixed).

* doc/lispref/modes.texi (Header Lines): Document it.
* lisp/display-line-numbers.el (header-line-indent): New variable.
(header-line-indent--line-number-width)
(header-line-indent--watch-line-number-width)
(header-line-indent--window-scroll-function): New helper functions.
(header-line-indent-mode): New minor mode.

* lisp/display-line-numbers.el (header-line-indent-width): New
variable.

* lisp/emacs-lisp/tabulated-list.el (tabulated-list-line-number-width)
(tabulated-list-watch-line-number-width)
(tabulated-list-window-scroll-function): Make into obsolete aliases.
(tabulated-list-mode): Use 'header-line-indent-mode'.

* lisp/emacs-lisp/tabulated-list.el (tabulated-list-init-header):
Adjust the header line format and computation.

* src/buffer.c (syms_of_buffer): Mention header-line-indent-mode.