Change DH_generate_parameters back to order 2q subgroup

For for G=2 and 5 DH_generate_parameters will continue to generate
the order 2q subgroup for compatibility with previous versions.

For G=3 DH_generate_parameters generates an order q subgroup, but it
will not pass the check in DH_check with previous OpenSSL versions.
Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9820)

CHANGES

@@ -50,8 +50,8 @@
      (CVE-2019-1552)
      [Richard Levitte]
 
-  *) Changed DH parameters to generate the order q subgroup instead of 2q.
-     Previously generated DH parameters are still accepted by DH_check
+  *) Changed DH_check to accept parameters with order q and 2q subgroups.
+     With order 2q subgroups the bit 0 of the private key is not secret
      but DH_generate_key works around that by clearing bit 0 of the
      private key for those. This avoids leaking bit 0 of the private key.
      [Bernd Edlinger]

crypto/dh/dh_gen.c

@@ -53,6 +53,10 @@ int DH_generate_parameters_ex(DH *ret, int prime_len, int generator,
  * for 2, p mod 24 == 23
  * for 3, p mod 12 == 11
  * for 5, p mod 60 == 59
+ *
+ * However for compatibilty with previous versions we use:
+ * for 2, p mod 24 == 11
+ * for 5, p mod 60 == 23
  */
 static int dh_builtin_genparams(DH *ret, int prime_len, int generator,
                                 BN_GENCB *cb)
@@ -83,13 +87,13 @@ static int dh_builtin_genparams(DH *ret, int prime_len, int generator,
     if (generator == DH_GENERATOR_2) {
         if (!BN_set_word(t1, 24))
             goto err;
-        if (!BN_set_word(t2, 23))
+        if (!BN_set_word(t2, 11))
             goto err;
         g = 2;
     } else if (generator == DH_GENERATOR_5) {
         if (!BN_set_word(t1, 60))
             goto err;
-        if (!BN_set_word(t2, 59))
+        if (!BN_set_word(t2, 23))
             goto err;
         g = 5;
     } else {