This plugin is intended to replace the setuidgid functionality of Mojo::Server.
It should be loaded in application startup and it will change the user and
group credentials of the process when Mojo::IOLoop is started, which occurs in
each worker process of a Mojo::Server::Prefork daemon like hypnotoad.

This allows an application to be started as root so it can bind to privileged
ports such as port 80 or 443, but run worker processes as unprivileged users.
However, if the application is not started as root, it will most likely fail to
change credentials. So, you should only set the user/group when the application
is started as root.
