The following env-vars have been renamed:
- YTCL_SERVER          to YTDL_SERVER
- YTCL_SERVER_USERNAME to YTDL_SERVER_USERNAME
- YTCL_SERVER_PASSWORD to YTDL_SERVER_PASSWORD

YTDL_SERVER is less confusing since it matches the name of ytdl-server.
Additionally, this change allows other programs to use the same env-vars
since the names are no longer tied to ytcl.

The old env-vars are still recognized, but they have been deprecated.
The new env-vars have precedence if both are defined.

Also added unit tests for the env-vars.