Port the code reading /proc/self/maps (and smaps) from Chromium

Originally reported on Google Code with ID 28

```
According to http://crbug.com/258451, the contents of /proc/self/maps cannot be read
atomically, and reading the file line by line may produce a garbled memory map.
We need to improve our /p/s/m reader by doing something similar to https://chromiumcodereview.appspot.com/18661009
```

Reported by `glider@google.com` on 2013-09-06 10:02:50
