Add g_autofree

The g_autoptr() being associated with the type name works out really
well for things like GHashTable.  However, it's a bit more awkward to
associate with "gchar".  Also because one can't use "char".
Similarly, there are a lot of other "bare primitive array" types that
one might reasonably use.

This patch does not remove the autoptr for "gchar", even though I
think it's rather awkward and strange.

Also while we're here, add a test case for the cleanup bits.

https://bugzilla.gnome.org/show_bug.cgi?id=744747

glib/docs.c

@@ -2422,6 +2422,40 @@
  * Since: 2.44
  */
 
+/**
+ * g_autofree:
+ *
+ * Macro to add an attribute to pointer variable to ensure automatic
+ * cleanup using g_free().
+ *
+ * This macro differs from g_autoptr() in that it is an attribute supplied
+ * before the type name, rather than wrapping the type definition.  Instead
+ * of using a type-specific lookup, this macro always calls g_free() directly.
+ *
+ * This means it's useful for any type that is returned from
+ * g_malloc().
+ *
+ * Otherwise, this macro has similar constraints as g_autoptr() - only
+ * supported on GCC and clang, the variable must be initialized, etc.
+ *
+ * |[
+ * gboolean
+ * operate_on_malloc_buf (void)
+ * {
+ *   g_autofree guint8* membuf = NULL;
+ *
+ *   membuf = g_malloc (8192);
+ *
+ *   /* Some computation on membuf */
+ *
+ *   /* membuf will be automatically freed here */
+ *   return TRUE;
+ * }
+ * ]|
+ *
+ * Since: 2.44
+ */
+
 /**
  * G_DEFINE_AUTOPTR_CLEANUP_FUNC:
  * @TypeName: a type name to define a g_autoptr() cleanup function for