Release resources associated with a signing or verifying handle.
||dkim_free() must be passed a handle returned by
||Releases resources that were allocated and assigned to the handle
dkim when it was returned by dkim_sign() or
dkim_verify(). The handle dkim may not be reused.
|| A DKIM handle returned by a previous call to dkim_sign()
- DKIM_STAT_OK -- operation was successful