void (*func)(void *user,
const unsigned char *param,
const unsigned char *value);
Defines a callback function to be used for providing signature tag/value
pairs to the calling program for local processing.
From within dkim_eoh(), each discovered
signature is assigned a DKIM_SIGINFO
handle. Using a callback, the caller can arrange to allocate caller-side
storage that might be desired by the caller for processing signatures; the
return value from that callback is assumed to be a pointer to such storage.
The callback defined here will be invoked once for each tag-value pair
found in each signature, thus providing that data to the caller for any
desirable local processing. The data thus provided are read-only.