The diocskerneldump_arg and netdump_conf (with embedded diocskerneldump_arg)
before it were copied in whole to the global nd_conf variable. After EKCD,
the diocskerneldump_arg contains sensitive key material. Before this
revision, de-configuring netdump would not remove the the key material from
global nd_conf.

Netdump doesn't have any use for the key data (that is handled in the core
dumper code), so just don't store it.

Unfortunately, I think this dates to the initial import of netdump in
r333283.