commandserver: add new forking server implemented without using SocketServer SocketServer.ForkingMixIn of Python 2.x has a couple of issues, such as: - race condition that leads to 100% CPU usage (Python 2.6) https://bugs.python.org/issue21491 - can't wait for children belonging to different process groups (Python 2.6) - leaves at least one zombie process (Python 2.6, 2.7) https://bugs.python.org/issue11109 The first two are critical because we do setpgid(0, 0) in child process to isolate terminal signals. The last one isn't, but ForkingMixIn seems to be doing silly. So there are two choices: a) backport and maintain SocketServer until we can drop support for Python 2.x b) replace SocketServer by simpler one and eliminate glue codes I chose (b) because it's great time for getting rid of utterly complicated SocketServer stuff, and preparing for future move towards prefork service. New unixforkingservice is implemented loosely based on chg 531f8ef64be6. It is monolithic but much simpler than SocketServer. unixservicehandler provides customizing points for chg, and it will be shared with future prefork service. Old unixservice class is still used by chgserver. It will be removed later. Thanks to Jun Wu for investigating these issues.