resolver: implement a way to specify config file for dir namespace

[markus2330]

In some applications (like lcdproc) it is possible to directly specify the configuration file (e.g. by command-line or environment variable). This is useful for running the application directly from where it has been built (without installing it) and also for testing.

In Elektra we could solve this, by passing some information to resolver (e.g. by environment variable). The dir namespace is ideally suited for this.

[markus2330]

duplicate of #1416