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: NDr. Stephen Henson <steve@openssl.org>

crypto/x509/x509_trs.c

@@ -285,7 +285,7 @@ static int trust_compat(X509_TRUST *trust, X509 *x, int flags)
 {
     /* Call for side-effect of computing hash and caching extensions */
     X509_check_purpose(x, -1, 0);
-    if (x->ex_flags & EXFLAG_SS)
+    if ((flags & X509_TRUST_NO_SS_COMPAT) == 0 && x->ex_flags & EXFLAG_SS)
         return X509_TRUST_TRUSTED;
     else
         return X509_TRUST_UNTRUSTED;

include/openssl/x509.h

@@ -199,8 +199,9 @@ DEFINE_STACK_OF(X509_TRUST)
 # define X509_TRUST_MAX          8
 
 /* trust_flags values */
-# define X509_TRUST_DYNAMIC      1
-# define X509_TRUST_DYNAMIC_NAME 2
+# define X509_TRUST_DYNAMIC      (1U << 0)
+# define X509_TRUST_DYNAMIC_NAME (1U << 1)
+# define X509_TRUST_NO_SS_COMPAT (1U << 2)
 
 /* check_trust return codes */