Retrieve the signature algorithm used to sign a message after verification.
||dkim_sig_getsignalg() is called after a signature is processed
either explicitly by a call to
dkim_sig_process() or by
dkim_eom() to get the signing
algorithm that was used to sign a message.
||A pointer to a dkim_alg_t which will receive
the algorithm that was used to sign the message.
||The function was called before dkim_sig_process() or
dkim_eom() processed the siganture.