Better initialization of random generator

Use full time precision to initialize random generator. The old
code was prone to initialize it to the same values in specific
circumstances (boot without RTC, multiple VMs starting at once).

sysdep/unix/io.c

@@ -2144,7 +2144,9 @@ io_init(void)
   // XXX init_times();
   // XXX update_times();
   boot_time = current_time();
-  srandom((uint) (current_real_time() TO_S));
+
+  u64 now = (u64) current_real_time();
+  srandom((uint) (now ^ (now >> 32)));
 }
 
 static int short_loops = 0;