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
Colin Walters