We need to get to the point of starting the session script before we
consider things a success. So this can fail in many different ways, not
just the daemonization. Adjust the error message to something more
generic to reflect this.
Some operating systems such as FreeBSD don't define a HOST_NAME_MAX
macro. The portable approach to determine the real host name limit is
calling sysconf(_SC_HOST_NAME_MAX) so do that instead.
This sets up a more correct session as there are key tasks that
need to be performed by PAM. E.g. systemd will allocate cgroups
and start base services.
In order to easily handle this as a system service the mapping of
displays is now done via a configuration file.