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

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

In other words, re-coalesce the two files,
lisp/progmodes/flymake-proc.el and lisp/progmodes/flymake-ui.el, back
into a single one, lisp/progmodes/flymake.el.

The changesets "Prefer HTTPS to FTP and HTTP in documentation" and
"allow nil init in flymake-allowed-file-name-masks to disable flymake"
are kept in place in the new lisp/progmodes/flymake.el.

This reverts Git commit eb34f7f5a29e7bf62326ecb6e693f28878be28cd.

Don't merge this back to master as development happening there builds
upon this work. See also
https://lists.gnu.org/archive/html/emacs-devel/2017-09/msg00932.html.