Codejail cannot run without a user set

If codejail is not configured with a user to run commands as, jailed code execution will fail. Effectively, this makes the user config option mandatory rather than optional.

This probably hasn't come up much because most users would configure it with a user. One possibility is to just make the option mandatory.

Details:

jail_code.py builds up a list to pass to subprocess.Popen. This is run as a command with arguments, rather than a shell command string. However, one of the arguments is 'TMPDIR=tmp', a construction to set an environment variable for a process in Bash. TMPDIR=tmp python ... works fine in Bash, but subprocess.Popen(['TMPDIR=tmp', 'python', ...]) does not work because TMPDIR=tmp is not a valid executable.

The reason it works with a user configured is that the command array will then first be prefixed with ['sudo', '-u', user]. The sudo command interprets the remainder of its arguments as a shell string rather than a command and arguments.

Start in jail_code.py and inspect how the command list is assembled before subprocess.Popen runs it, especially when no user is configured. Reproduce jailed execution with the user option unset, then determine whether the intended resolution is to correct the command construction or make user mandatory. Done means the chosen behavior works consistently and is covered by the relevant existing tests.