Using device path instead of mount point for ecryptfs mounts

Basically with this patch I'm trying to fix this issue I opened earlier: #19

However it was a bit more tricky than the solution I proposed in that post.

1. Device paths of ecryptfs mounts can be symbolic links and these symlinks need to be followed.
2. I added a routine that checks if both the encrypted and decrypted files are inside the directory that is about to be analysed. Otherwise baobab might end up analysing neither of them.

Compiled and tested on Arch Linux. Worked fine for me.