Fix MAC() device path printing.

67491a308 accidentally made it print MAC(MAC( .

Signed-off-by: Peter Jones <pjones@redhat.com>
Make the optional_data_len size signed.

Steve McIntyre's testing on i386 with efivars using the legacy path
(i.e. not efivarfs) also found a bug here - we're checking
optional_data_len for negative error values, but it's unsigned.  We're
not going to have 4GB of optional loader data, so just make it signed.

This should fix Issue #21 .

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