Fix `setuid` feature

The call to setuid() was previously after the simulator and verifier
processes were spawned, so both those processes always ran as the user
who started the process (which would have been `root` if the setuid
feature was being used), rather than the setuid user. In particular,
this meant that the >>> battle dev console could be used to execute
arbitrary code as root (if the server was started as root).

This commit makes the setuid() feature work as advertised, but it
still should not be used for the reasons given in config-example.js.
Instead of using setuid, start the server as an unprivileged user and
use port forwarding (e.g. iptables ) to use the port(s) you prefer.