Include self-signed flag in certificates.

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.

(Imported from upstream's c00f8d697aed17edbd002e2f6c989d8fbd7c4ecf)