Support attaching to an existing process

[JoshMcguigan]

Currently backlight only supports tracing processes that it spawns. It should be expanded to support attaching to existing processes. I think this should mostly "just work", with the main new function being reading the existing mmap'd libraries and resolving symbols from them. As of now, since backlight is tracing a process from the very start, it assumes it will see the mmap system call for each library as it is loaded.