FIX: Error Logging of Library Loading Mechanics

Following the ticket [CVM-595](https://sft.its.cern.ch/jira/browse/CVM-595) I changed the error handling around `dlopen()` to produce more useful output. Unfortunately `dlopen()` only returns `NULL` on error and doesn't set `errno`.

*Note:* Adds also two utility functions: `SymlinkExists()` and `IsAbsolutePath()` + test.