PATH management is quite messy

On several occasion, we are calling some external programs. The paths are hardcoded where it should rely on whatever is set in $PATH environment variables.

This leads to crash on *BSD+ports and MacOS+brew or macports as the bins are usually found in /usr/local.

Also the get_path pattern is repeated in several files (check possible paths + checking if we can execute). This needs to be factorized.