add default logdir control to deployment API

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on August 28, 2012 12:41:47_

dr_config.c sets the logdir to the root DR dir plus /logs

I'm adding a -logdir runtime option for Dr. Memory but:

```
/* XXX: should we expose this in the dr_register_process() params (and thus
* dr_process_is_registered() and dr_registered_process_iterator_next())?
* We now have a -logdir runtime option so we don't need to expose it for full
* functionality anymore but it would serve to reduce the length of option
* strings to have more control over the default.
* Linux dr{config,run} does allow such control today.
*/
```

xref https://code.google.com/p/drmemory/issues/detail?id=874 another option would be to demote logdir from a first-class config var and
have a default inside the core itself. that seems cleaner from some
perspectives but the option string length and historical product
reasons (logdir is not just for debugging logs) are reasons to keep as a
config var.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=886_