Minimal tweak as an attempt to future-proof Flymake API

Minimal tweak as an attempt to future-proof Flymake API

Discussed with Stefan that this should allow Flymake to request more
from backends in the future, while also allowing backends to report
more accurately.

* lisp/progmodes/elisp-mode.el (elisp-flymake-checkdoc)
(elisp-flymake-byte-compile): Adjust to new API.

* lisp/progmodes/flymake-proc.el ()
(flymake-proc-legacy-flymake): Adjust to new API.

* lisp/progmodes/flymake.el (flymake-diagnostic-functions):
Review API again.
(flymake--handle-report): Allow other keys.  Change ACTION to
REPORT-ACTION.