New Flymake rewrite in emacs-26

[João Távora]

Hi,

As discussed, I have just merged the Flymake rewrite into emacs-26. I
did this sooner than I expected so people can give it as much testing as
possible.

`flymake-mode' is not active by default, so when writing Elisp, I
encourage you to try out the new backends. Just M-x flymake-mode or

  (add-to-list 'emacs-lisp-mode-hook 'flymake-mode).

Of course if you were already using Flymake for C, C++, etc. Everything
should also still work (slightly better, in fact). Let me know if it
doesn't.

Some things worth noting:

* There are two older commits in emacs-26 that must be merged to master
  after all, otherwise the merge-back of this feature won't work.

  This is because there was a reversion of the decision to bring this
  feature to emacs-26 (see
  https://lists.gnu.org/archive/html/emacs-devel/2017-09/msg01020.html
  and
  https://lists.gnu.org/archive/html/emacs-devel/2017-09/msg01030.html)
  the reversion

  The two commits are

  ce540f8a68..: João Távora 2017-09-27 Revert "Split flymake.el into 
flymake-proc.el and flymake-ui.el"
  7cf59c6635..: João Távora 2017-09-27 Revert "Add flymake-backends defcustom"

* The most important docstrings are in place, but the Texinfo
  documentation (which I also rewrote considerably) is missing the bits
  about the new API for writing backends.

  Ideally, I would like some feedback on it (besides Stefan, of
  course). Please see the docstrings of `flymake-diagnostic-functions'
  and `flymake-diagnostic-types-alist'.

  Even more ideally, someone would write a nicer backend for Emacs's C
  sources (the current "legacy" one requires Makefiles to contain a
  "syntax-check" target, which isn't practical, I presume)

Thanks,
João