Make tor_check_port_forwarding more generic
tor_check_port_forwarding() in util.c contains both code specific to tor-fw-helper as well as generic code for launching a child process. It might be useful to make this more generic so it could be used for launching other processes.
On this topic, in bug #1903 (moved), nickm said:
It seems that the code here does two things: one is a general task "Launch a child process and see what it says" and another is a more specific task "Launch a tor-fw-helper instance and act based on its output.)" It might be a good idea to disentangle these eventually, in case we ever want to launch anything else.
It might be cleaner to move the static variables from tor_check_port_forwarding into some kind of struct, in case we ever want to launch two things in the future.