This commit adds the option 'services.syslogd.defaultConfig', which is
the main syslog.conf file used by the daemon. Like before, That file can
be extended by means of 'services.syslogd.extraConfig'. Users who want a
completely different configuration, however, can re-define defaultConfig
to their liking.
Furthermore, the option services.syslogd.tty' is now optional: setting
its value to the empty string "" disables logging to TTY altogether.
svn path=/nixos/trunk/; revision=27769