Add option to use SCREEN instead of X for remote console

[rsnow]

This patch changes dw-start.sh to give the option to use SCREEN as the execution display. SCREEN is a text based terminal multiplexer (think lxterminal for text, with tabs and the like) and works great for detached/remotely running applications, without the overhead of X, and then attaching as needed via SSH or the like. If you have screen installed, you can specify it in the dw-start.sh script in the USESCREEN variable at the top of the file.

If USESCREEN is specified, then after the 30 second wait, the script will check to see if there is an existing SCREEN session named "direwolf", if not then it starts it in the background. Next, direwolf is started in its own screen in side of the "direwolf" screen session.

To attach to this screen remotely, you can ssh in and then execute "screen -S direwolf -D -RR" and monitor direwolf. If you want to detach, you can use C-x C-d to detach and you can then reattach later or leave it running.

I moved a few items around because the screen section will need some of those variables sooner than the X stuff that is left alone at the bottom.

Tested on my RPi from command line and through reboots.

[rsnow]

Removing this pull request, it's outdated. I've worked extensively on dw-start.sh and it does much more now.