Add a minor mode interface for display-line-numbers
* lisp/cus-start.el: Use the new display-line-numbers group.
* lisp/display-line-numbers.el: New file.
* doc/emacs/custom.texi (Init Rebinding): Re-add entry that used to
belong to linum-mode.
* doc/emacs/modes.texi (Minor Modes): Summarize the mode.
* etc/NEWS: Document display-line-numbers-mode and its customization
variables, and mention that display-line-numbers-width is
buffer-local.
* src/xdisp.c (syms_of_xdisp) <display-line-numbers-width>: Fix a
typo.