* Modularised the xinetd service. tftp has been factored out into a

  separate module, which just declares a configuration value that
  causes the xinetd module to add it to xinetd.conf.  Also Nixified
  the service declarations to abstract over the inetd implementation.
  
* Renamed the services.xinetd.tftpd options to services.tftpd.  The
  fact that the tftpd module uses xinetd is an implementation detail.

* xinetd: use -dontfork to let Upstart monitor it, and use -syslog to
  get error messages at startup.

svn path=/nixos/trunk/; revision=16803