Fix filesystem littering by Flymake's legacy backend

Fix filesystem littering by Flymake's legacy backend

The Flymake legacy "proc" backend, which is active by default will try
to syntax-check foo.c/foo.cpp and many other types of files, but on
failing to find a suitable Makefile target, will fail.  There's
nothing wrong with that except that it used to leave behind the
foo_flymake.c and foo_flymake.cpp auxiliary files behind, littering
the filesystem.

* lisp/progmodes/flymake-proc.el (flymake-proc-legacy-flymake):
Call init-function inside of the unwind-protect.