Add Coccinelle usage: one for blacklisting, one for patch collection

This is inspired by the [Coccinelle](http://coccinelle.lip6.fr/) usage
in systemd.  I also took it a bit further and added infrastructure
to have spatches which should never apply.  This acts as a blacklist.

The reason to do the latter is that coccinelle is *way* more powerful than the
regular expresssions we have in `make syntax-check`.

I started with blacklisting `g_error_free()` directly. The reason that's bad is
it leaves a dangling pointer.

Closes: #754
Approved by: jlebon

tests/coccinelle/README.md

@@ -0,0 +1,2 @@
+Add patches here which should never match in the code; i.e. the suggested
+replacement may be junk.

tests/coccinelle/raw-free.cocci

@@ -0,0 +1,5 @@
+@@
+expression p;
+@@
+- g_error_free (p);
++ g_clear_error (&p);