Export the 64-bit partition offset and use it in chain.c

Export the 64-bit partition offset and use it in chain.c

When used with the "fs" option to chain.c32, we need to know our own
filesystem offset.  That means knowing if we used the MBR vs GPT
partition information, as well as if we ended up using the passed-in
information or not.

Resolve this by providing an explicit pointer to the current partition
offset.  Eventually this should be replaced by some kind of statfs()
call.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>