common: add autofree helper

Much like the auto-closing helper for file descriptors, add a new
auto-free helper that is meant to be used with dynamically allocated
memory, a la:
  autofree char *data = NULL;
  ...
  data = malloc(size);
When data goes out of scope, cleanup_free will be called with &data,
i.e. cleanup_free(&data), which in turn will call free(3) data. In
order to work with all types, e.g. 'char *' (resulting in char **
being passed to cleanup_free) or 'int *' (resulting in int ** being
passed to cleanup_free), cleanup_free is defined to work with void *,
hence the odd-looking cast: void *target = *(void **) ptr.

common/common-helpers.c

@@ -39,3 +39,4 @@ POSSIBILITY OF SUCH DAMAGE.
  * a specific call to the function the linker will expect an definition.
  */
 extern inline void cleanup_close(int *fd);
+extern inline void cleanup_free(void *ptr);