Make sure we initialize all member variables in HostInfo.
Check arguments to DS2LOG.

We use __attribute__((__printf__())) for this. This commit also contains
a few fixes for errors that showed up when we started using this
attribute.
Make a copy of the input va_list in vLog before passing it.

We were calling vsnprintf() with the same va_list multiple times when
our temporary buffer was too small. We need to make a copy first.