Add various improvements to the "pre-flight installer":

Add various improvements to the "pre-flight installer":

- Add pfi_autologin variable.  If set to a valid username, the
  installation medium boot process will automatically log in as
  that user.  Defaults to "NONE".

- Add pfi_shutdown_command variable.  This can be set to the
  desired command used to shut down the system when the installer
  finishes doing its thing.  Defaults to "shutdown -h now".

- Launch thttpd on boot if pfi_frontend="cgi".

  In combination, these three settings allow completely
  headless (no monitor, no keyboard) operation of the installer
  independent of a serial connection (i.e. over the network.)

- Have the pfi_sshd_permit_root_login variable honour
  "without-password" and "forced-commands-only" settings. [1]

[1] Submitted by: geekgod