chutney start could tell users how to kill old chutney tors

The most common reason chutney start fails to start all tor processes is port conflicts caused by old chutney tor processes still running.

We could tell users how to kill processes (should we?) using a summary of the following information from https://trac.torproject.org/projects/tor/wiki/doc/TorChutneyGuide

"However, sometimes chutney fails to launch due to a persistent error. The most common issue is having a previous chutney network active. Old chutney networks are typically killed by chutney/tools/bootstrap-network.sh using chutney stop, but sometimes this doesn't happen. The tor processes launched by chutney can be found using a command like ps auxwww | grep chutney | grep tor. Ensure each pid doesn't belong to a process you want, like a system tor or an unrelated process, then kill each chutney tor pid using kill. Alternately, each tor instance's pid files can be found in the chutney/net/nodes directory. (Old nodes directories are renamed using an increasing numeric timestamp as a suffix.)"

Use:

• chutney stop
• ps auxwww | grep chutney | grep tor and carefull check each pid before using kill on it
• the tor pid files in the chutney/net/nodes* directories (maybe, these can be very old)