digsig: changed type of the timestamp

time_t was used in the signature and key packet headers,
which is typedef of long and is different on 32 and 64 bit architectures.
Signature and key format should be independent of architecture.
Similar to GPG, I have changed the type to uint32_t.

Signed-off-by: Dmitry Kasatkin <dmitry.kasatkin@intel.com>
Signed-off-by: James Morris <jmorris@namei.org>