dmd: make user-mode easier to use.

[Alex Sassmannshausen <alex.sassmannshausen@gmail.com>]

Hello,

This patch series allows dmd to work out of the box in 'user-mode'
after installation. To achieve this it ensures all dmd related files
are stored in ~/.dmd.d/ when run in user-mode, and it creates a
bare-bones configuration file for the user to start building upon if
no configuration file exists yet.

I decided to implement the above minimising any changes to the actual
logic of existing dmd. The result is a slight mixing up of side-effect
folder/file creation and side-effect free checks.

The alternative would have been to separate the folder creation from
the procedures carrying out the checks for the existence of the
configuration folder and files.

I wasn't sure whether it was better to minimise the changes in the
patches or better to 'just do the right thing', even if it entails
larger and more complex patches. Let me know your thoughts if you
think I should have gone the other way.

Best wishes,

Alex