Remember suitable hash function for any signature algorithm.

This commit changes `ssl_parse_signature_algorithms_ext` to remember
one suitable ( := supported by client and by our config ) hash
algorithm per signature algorithm.

It also modifies the ciphersuite checking function
`ssl_ciphersuite_match` to refuse a suite if there
is no suitable hash algorithm.

Finally, it adds the corresponding entry to the ChangeLog.
diff --git a/library/ssl_ciphersuites.c b/library/ssl_ciphersuites.c
index 3b25182..c1a92d6 100644
--- a/library/ssl_ciphersuites.c
+++ b/library/ssl_ciphersuites.c
@@ -1817,6 +1817,23 @@
             return( MBEDTLS_PK_NONE );
     }
 }
+
+mbedtls_pk_type_t mbedtls_ssl_get_ciphersuite_sig_alg( const mbedtls_ssl_ciphersuite_t *info )
+{
+    switch( info->key_exchange )
+    {
+        case MBEDTLS_KEY_EXCHANGE_RSA:
+        case MBEDTLS_KEY_EXCHANGE_DHE_RSA:
+        case MBEDTLS_KEY_EXCHANGE_ECDHE_RSA:
+            return( MBEDTLS_PK_RSA );
+
+        case MBEDTLS_KEY_EXCHANGE_ECDHE_ECDSA:
+            return( MBEDTLS_PK_ECDSA );
+
+        default:
+            return( MBEDTLS_PK_NONE );
+    }
+}
 #endif /* MBEDTLS_PK_C */
 
 #endif /* MBEDTLS_SSL_TLS_C */