Split Flymake into flymake.el into flymake-proc.el (again!)

Split Flymake into flymake.el into flymake-proc.el (again!)

After deciding that this work would continue on master only, which
caused two commits named

   Revert "Split flymake.el into flymake-proc.el and flymake-ui.el"

and

   Revert "Add flymake-backends defcustom"

to be added to the emacs-26 branch, further discussion reversed that
decision.

   See:

     https://lists.gnu.org/archive/html/emacs-devel/2017-09/msg01020.html
     https://lists.gnu.org/archive/html/emacs-devel/2017-09/msg01030.html

This means that those two commits MUST be merged to master AFTER ALL.

flymke-proc.el contains the main syntax-checking backend, while
flymake.el keeps mostly the UI part.

* lisp/progmodes/flymake-proc.el: New file.  Require flymake.

* lisp/progmodes/flymake.el: Require flymake-proc.el at the end.