#7 Add overlay directory structures

[Szasz Pal]

Allow users to use overlay directory structures, ie
specifying a different set of profile files to be used
in addition to the normal one. The reasons:
- some users might have some confidential information
(not a good thing to have, but it might be possible)
- some users expect a different value for some
variables. They could put those on the overlay db hence
it won't get lost when the main db is updated
- some users are experiencing with new profiles, and
they don't want to upload until it's tested
- etc...