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

coccinelle/README.md

@@ -0,0 +1,6 @@
+This is a directory of semantic patches
+to apply with coccinelle, like the collection in systemd:
+https://github.com/systemd/systemd/tree/29f32655842a0712e8db482bcefc4da8908460c8/coccinelle
+
+See also the tests/coccinelle directory which
+has spatches which detect errors.

coccinelle/newstyle.cocci

@@ -0,0 +1,22 @@
+@@
+expression p;
+@@
+- glnx_set_error_from_errno (p);
+- goto out;
++ return glnx_throw_errno (p);
+@@
+expression p;
+@@
+- if (!p)
+-   goto out;
++ if (!p)
++   return FALSE;
+@@
+expression p;
+@@
+- gboolean ret = FALSE;
+...
+- ret = TRUE;
+- out:
+- return ret;
++ return TRUE;