Calculate total physical memory on all platforms

_From [billiej...@gmail.com](https://code.google.com/u/107489592441517285058/) on March 25, 2009 18:45:56_

```
Implement a get_total_phymem() function or a TOTAL_PHYMEM constant to read
the total physical memory available.
```

_Original issue: http://code.google.com/p/psutil/issues/detail?id=46_
