Better sign-up and sign-in

[tilmankamp]

On sign-up the user

• should be asked where to store the .pituser.txt file,
• should get informed where the .pituser.txt file was saved.

On sign-in the user should know which .pituser.txt file is about to be used with which user name.

There should be extra commands for signing-out and signing-in that both ask the user (options if in doubt) for the location of the .pituser.txt file.