Include self-signed flag in certificates by checking SKID/AKID as well

Include self-signed flag in certificates by checking SKID/AKID as well
as issuer and subject names. Although this is an incompatible change
it should have little impact in pratice because self-issued certificates
that are not self-signed are rarely encountered.
(cherry picked from commit b1efb7161f409c81178b9aa95583db3390f90b1b)