Misc. minor adjustments to Flymake

Misc. minor adjustments to Flymake

- Add a half-decent minor-mode menu;
- Fix "waiting for backends" mode line message;
- Adjust the flymake-diag-region API;
- Autoload the flymake-log macro;
- Auto-disable the legacy backend in more situations;
- Fix a couple of warnings in legacy backend.

* lisp/progmodes/flymake-proc.el
(flymake-proc--diagnostics-for-pattern): Use new
flymake-diag-region.

* lisp/progmodes/flymake-proc.el
(flymake-proc-legacy-flymake): Do error when no
buffer-file-name or not writable.
(flymake-proc-legacy-flymake)
(flymake-proc-simple-cleanup): Don't reference flymake-last-change-time

* lisp/progmodes/flymake.el (flymake-diag-region):
Autoload.  Take buffer as first argument.

* lisp/progmodes/flymake.el (flymake-switch-to-log-buffer):
New command.
(flymake-menu): Add a simple menu.
(flymake--mode-line-format): Use menu.  Fix message.  Switch to
log buffer when clicking exceptional warnings.