Use tls_choose_sigalg for client auth.

For client auth call tls_choose_sigalg to select the certificate
and signature algorithm. Use the selected algorithm in
tls_construct_cert_verify.

Remove obsolete tls12_get_sigandhash.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2643)
diff --git a/ssl/statem/statem_clnt.c b/ssl/statem/statem_clnt.c
index 8ca3c4c..4923e24 100644
--- a/ssl/statem/statem_clnt.c
+++ b/ssl/statem/statem_clnt.c
@@ -3090,10 +3090,8 @@
  */
 static int ssl3_check_client_certificate(SSL *s)
 {
-    if (!s->cert || !s->cert->key->x509 || !s->cert->key->privatekey)
-        return 0;
     /* If no suitable signature algorithm can't use certificate */
-    if (SSL_USE_SIGALGS(s) && !s->s3->tmp.md[s->cert->key - s->cert->pkeys])
+    if (!tls_choose_sigalg(s, NULL) || s->s3->tmp.sigalg == NULL)
         return 0;
     /*
      * If strict mode check suitability of chain before using it. This also