Final excision of libgsystem dependency

Lots and lots of preparation led to this moment - when nothing
apparent changes for users!  Woo!

But seriously, having the extra dependency is a minor annoyance, and
in the big picture I think the libgsystem idea was wrong - we need to
land things in GLib, and use git submodules for API-unstable or
Linux-specific sharing.  For a lot of OSTree, the libgsystem `GFile*`
orientation was also wrong, we really want fd-relative.

Closes: #444
Approved by: jlebon

src/libotutil/otutil.h

@@ -23,9 +23,7 @@
 #pragma once
 
 #include <gio/gio.h>
-#include <libgsystem.h>
 #include <string.h> /* Yeah...let's just do that here. */
-#include <gsystem-local-alloc.h>
 #include <libglnx.h>
 
 #define ot_gobject_refz(o) (o ? g_object_ref (o) : o)