Make it possible to check for explicit auxiliary trust

By default X509_check_trust() trusts self-signed certificates from
the trust store that have no explicit local trust/reject oids
encapsulated as a "TRUSTED CERTIFICATE" object.  (See the -addtrust
and -trustout options of x509(1)).

This commit adds a flag that makes it possible to distinguish between
that implicit trust, and explicit auxiliary settings.

With flags |= X509_TRUST_NO_SS_COMPAT, a certificate is only trusted
via explicit trust settings.

Reviewed-by: Dr. Stephen Henson <steve@openssl.org>