Fix markup of app_demo_flat.xml (Network Guide)

My changes are mainly:
* use userinput instead of computeroutput as appropriate
* add prompts
* use programlisting (only using language="bash" for shell code)
* use filename

Change-Id: I1ae3f45ae24676d6f6e09cfef6497f5fb7a4cb99
Merge "Fix markup of app_demo_flat.xml (Network Guide)"