Change timestamp fields from signed to unsigned long.

* src/gpgme.h.in (struct _gpgme_subkey): Chnage timestamp and expires.
(struct _gpgme_key_sig): Ditto.
(struct _gpgme_new_signature): Ditto.

Note that unsigned long has always been used for gpgme_signature_t.
A change to time_t would be a real ABI change and require may users of
gpgme to adjust their code. The signed/unsigned change is sufficient
and mostly invisible.