Fix authvar

Those are the patches to make authvar work. I've tested on OVMF by copying the signed file to /sys/firmware/efi/efivars and the db updated immediately.

example of commands:
authvar -s -N security -S MyPK -n db -d certdb -f db-new.esl -e db-d719b2cb-3d3a-4596-a3bc-dad00e67656f

I'm not sure about the intention of "import", so I left it as unimplemented.