const char * config)
Passes arbitrary configuration information to the resolver.
||dkim_dns_config() can be called at any time, but is
presumably most useful prior to doing any DNS operations such as are done
during dkim_eoh() when verifying.
||The library instantiation handle, returned by
||A null-terminated string containing configuration data for the
resolver. The string is passed intact to the resolver and is not
used by this library.
- DKIM_DNS_SUCCESS -- successful operation (the underlying
configuration function returned something other than zero)
- DKIM_DNS_ERROR -- an error occurred