Add flymake-backends defcustom

Add flymake-backends defcustom

* lisp/progmodes/flymake-proc.el (flymake-proc-can-syntax-check-buffer):
Rename from flymake-can-syntax-check-file.  Suitable for adding to
flymake-backends.
(flymake-proc-start-syntax-check): Rename from
flymake-start-syntax-check.  Don't check again if buffer can be
checked.
(add-to-list flymake-backends): Hook only flymake-ui.el

* lisp/progmodes/flymake-ui.el (flymake-backends): New
defcustom.
(flymake-on-timer-event, flymake-after-change-function)
(flymake-after-save-hook, flymake-find-file-hook): Call new
flymake--start-syntax-check-buffer and
flymake--can-syntax-check-buffer.
(flymake-mode): Call flymake--can-syntax-check-buffer and set
flymake-backend.
(flymake--backend): New buffer-local variable.