Process track: handle orphan process as well

[cappe987]

Right now the process tracking stops when the main process exits. Any orphans will therefore not be monitored. Add flag (maybe -o|--orphans) that lets Capmon run until all child processes have exited.

Suggested implementation:
When waitpid indicates the process has exited, look in pid_tree for the next PID to wait for. Alternatively, keep a separate linked list of PIDs where you add everything that is added to pid_tree. When one PID indicates it has exited, remove it from the list and try the next. If the linked list is empty then all children have exited.