common: Don't assume on-disk layout matches in-memory layout.

* g10/packet.h (PKT_signature): Change revkey's type from a struct
revocation_key ** to a struct revocation_key *.  Update users.

revkey was a pointer into the raw data. But, C doesn't guarantee that
there is no padding. Thus, we copy the data.

• Signed-off-by: Neal H. Walfield <neal@g10code.com>.