[Startup] Log output of startup script

This adds a 'teeOutput' function to tee the output instead of just
logging to the log file as the 'logOutput' function does.
[Startup] Networking tweaks

- Mark eth0 down when it does not have link
- Allow setting 'ENABLED=0' in interface config file to disable an
  interface.  There is no UI for this, it is mainly for testing, but
  adding a UI could be done later if desired.
- Don't bring up the fallback IP on eth0 if there is no link
- Don't bring up the fallback IP on eth0 if there is a static IP
  configured.