Add some sample code for efi_get_next_variable_name()

Signed-off-by: Peter Jones <pjones@redhat.com>
Add documentation for efi_get_next_variable_name()

Signed-off-by: Peter Jones <pjones@redhat.com>
API change, so bump version to 0.4

Signed-off-by: Peter Jones <pjones@redhat.com>