Reject when explicit trust EKU are set and none match.

Returning untrusted is enough for for full chains that end in
self-signed roots, because when explicit trust is specified it
suppresses the default blanket trust of self-signed objects.

But for partial chains, this is not enough, because absent a similar
trust-self-signed policy, non matching EKUs are indistinguishable
from lack of EKU constraints.

Therefore, failure to match any trusted purpose must trigger an
explicit reject.

Reviewed-by: Richard Levitte <levitte@openssl.org>