*BIG* verify code reorganisation.

The old code was painfully primitive and couldn't handle
distinct certificates using the same subject name.

The new code performs several tests on a candidate issuer
certificate based on certificate extensions.

It also adds several callbacks to X509_VERIFY_CTX so its
behaviour can be customised.

Unfortunately some hackery was needed to persuade X509_STORE
to tolerate this. This should go away when X509_STORE is
replaced, sometime...

This must have broken something though :-(