Adds threads.c example
Improves verbose output
Renames Process#pid to tid

These are thread IDs, I will probably have to stores PIDs (process ids =
thread group ids) soon.
Stores tgid in struct Process
Correctly handles execve() from non-thread-leader

On Linux, if a non-leader thread calls execve(), the thread leader will
complete that call and the initial thread will disappear without a
trace. Other threads are terminated non-silently.
Adds threads.c example to tests/run.py