tweak printout for holdout

Hi John, here is the tweak for holdout printout, as requested in the mailing list. 

Now the printout looks like ( when holdout loss and holdout example counter are both 0.)
```
Num weight bits = 18
learning rate = 0.5
initial_t = 0
power_t = 0.5
decay_learning_rate = 1
creating cache_file = test_data.cache
Reading datafile = test_data
num sources = 1
average    since         example     example  current  current  current
loss       last          counter      weight    label  predict features
4.710480   4.710480            3         3.0   1.0000   0.6369        5
 unknown    unknown            6         6.0   3.0000   1.9788        7 h
 unknown    unknown           11        11.0   1.0000   1.7617        5 h
 unknown    unknown           22        22.0   3.0000   2.9151        7 h

finished run
number of examples per pass= 4
passes used = 10
weighted example sum = 40
weighted label sum = 90
average loss = 0.736859
best constant = 2.25
total feature number = 240
```
If this looks fine, I will update the printout for other reductions, as well as updating the stderr files in the test cases.