Make Flymake mode-line indicator customizable (bug#33740)

Make Flymake mode-line indicator customizable (bug#33740)

* lisp/progmodes/flymake.el (flymake-mode): Use
flymake-mode-line-format.
(flymake--mode-line-format): Remove.
(flymake-mode-line-counter-format, flymake-mode-line-format): New
defcustom.
(flymake-mode-line-title, flymake-mode-line-exception)
(flymake-mode-line-counters, flymake-error-counter)
(flymake-warning-counter, flymake-note-counter): New variables.
(flymake--mode-line-title, flymake--mode-line-exception)
(flymake--mode-line-counters, flymake--mode-line-counter): New
helpers.

* doc/misc/flymake.texi (Customizable variables): Mention
flymake-mode-line-format and flymake-mode-line-counter-format

* etc/NEWS: Mention Flymake's customizable mode-line.