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.

stderr has been redirected to /dev/null so all logging needs to go to
syslog.

This is what display managers do, so it is expected by many
environments.

We terminated the child instead of the parent after fork().

Reported by Jan Grulich from Red Hat.

Not sure why this got in there, but it is incorrect as it results in
creating files with too liberal permissions.

This makes our builds directly compatible with most distributions
without packagers/users having to specify extra flags.

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.