backporting Peter's fix for better RAND() initialization from 4.0

sql/sql_class.cc:
  backporting Peter's fix for better RAND() initialization from 4.0
  to see what it does try
  perl -e 'while($N=shift) { printf "N=%-10d",$N; @b=@c=(); for (1..$N) { @A=split(" ",`mysql -BNe "select rand(), rand(), rand(), rand()"`); for(0..3) { $b[$_]+=$a[$_]; $c[$_]+=$a[$_]*$a[$_]; }} for (0..3) { printf "%15.14f ",($c[$_]-$b[$_]*$b[$_]/$N)/$N } print "\n" }' 10 10 10 100 100 10 100 1000
  before and after the fix

sql/sql_class.cc

@@ -156,7 +156,7 @@ THD::THD():user_time(0),fatal_error(0),last_insert_id_used(0),
   */
   {
     pthread_mutex_lock(&LOCK_thread_count);
-    ulong tmp=(ulong) (rnd(&sql_rand) * 3000000);
+    ulong tmp=(ulong) (rnd(&sql_rand) * ((ulong)~0L));
     randominit(&rand, tmp + (ulong) start_time,
 	       tmp + (ulong) thread_id);
     pthread_mutex_unlock(&LOCK_thread_count);