SCT signature verification, encode/decode and accessor functions

Previously, I'd put the n2l8 macro in ssl_locl.h, and it operated on BN_ULLONG.  But this broke compilation of 1.0.2-beta1 on various platforms.  Since it was Steve who, to fix this, added the #define SCT_TIMESTAMP stuff and moved the (modified) n2l8 macro to v3_scts.c, do you want to check with him about changing this to a typedef?
At the time, I asked Steve why he didn't just define BN_ULLONG on the platforms that don't currently define it.  He replied: "It was the easiest option. Defining BN_ULLONG has an effect on the bn routines and I didn't want to risk breaking them."