Use /proc//root for remote unwinding

[Mikhail Durnev]

Dear libunwind maintainers,

I will highly appreciate if you take a look into my patch on GitHub: 
https://github.com/libunwind/libunwind/pull/195

When we are backtracing processes running in a container, we cannot rely 
on ELF paths in /proc/[pid]/maps. Those paths are valid inside the 
container. We have to add /proc/[pid]/root to each path to access the 
ELF files from outside of the container.

Thanks,
Mikhail