Rename flymake--backend-state to flymake--state

Rename flymake--backend-state to flymake--state

The previous name was confusing and akward and dreadful to type and
read.

* lisp/progmodes/flymake.el (flymake--state): Rename from
flymake--backend-state.
(flymake--with-backend-state): Use flymake--state.
(flymake--handle-report): Use flymake--state.
(flymake--collect): Use flymake--state.
(flymake-running-backends): Use flymake--state.
(flymake--disable-backend): Use flymake--state.
(flymake--run-backend): Use flymake--state.
(flymake-start): Use flymake--state.
(flymake-mode): Use flymake--state.
(flymake--mode-line-title): Use flymake--state.
(flymake--mode-line-exception): Use flymake--state.
(flymake--mode-line-counter): Use flymake--state.