SCT signature verification, encode/decode and accessor functions

If I change SCT_get0_logid so that it can return NULL if "not set", then I guess we should be consistent and have a "not set" concept for each and every SCT attribute.
I think I'll change this function to...
int SCT_get0_version(const SCT *sct);
...and let it return the SCT version number or -1 if "not set".