void * (*func)(void *closure));
Defines a callback function to be used for allocating local data storage
to be associated with signatures.
From within dkim_eoh(), each discovered
signature is assigned a DKIM_SIGINFO
handle. If this callback function is defined, it will be called once for each
such handle to allocate any caller-side storage that might be desired by
the caller; the return value is assumed to be a pointer to such storage,
and can be retrieved later by the caller during signature processing.