Gio::File: Add measure_disk_usage() and measure_disk_usage_async().

* gio/src/file.[hg|ccg]: Wrap g_file_measure_disk_usage(),
  g_file_measure_disk_usage_async() and
  g_file_measure_disk_usage_finish(), based on the copy() and
  copy_async() methods, which also have 2 slots.
* tools/m4/convert_glib.m4: Add a conversion for guint64&.
Spawn: Use the new SPAWN_DEFAULT instead of 0.

* glib/src/spawn.hg: The value is the same, however.