Section 11.1 of RFC4880 requires that binding signatures on
signing subkeys contain a valid embedded signature that cross-certifies
the primary key. This is to avoid the weakness described at
https://www.gnupg.org/faq/subkey-cross-certify.htmlFixes#10740
Change-Id: Ibe039662497832945957b001a83080ba29213703
Reviewed-on: https://go-review.googlesource.com/9799
Reviewed-by: Adam Langley <agl@golang.org>