Flymake: Experimental 'fancy' flymake-show-diagnostics-at-end-of-line

Flymake: Experimental 'fancy' flymake-show-diagnostics-at-end-of-line

* doc/misc/flymake.texi (Customizable variables): Describe new
flymake-show-diagnostics-at-end-of-line.

* etc/NEWS: Mention 'flymake-show-diagnostics-at-end-of-line'.

* lisp/progmodes/flymake.el
(flymake-show-diagnostics-at-end-of-line): Update docstring.

* lisp/progmodes/flymake.el (flymake--eol-overlay-summary):
Rework.  Move to separate section.
(flymake--update-eol-overlays): Rework.  Use 'display'.  Move to
separate section.
(flymake--eol-draw-fancy-1, flymake--eol-draw-fancy): New helpers.
(flymake-end-of-line-diagnostics-face): Remove box.

(cherry picked from commit 0262e3e158e35770f864d3131baf8f1793a20e58)