Make Pliant easier to install anywhere in the disk

Pliant uses three directories:

The first one is named 'pliant_root_path' within Pliant code.
It is computed in 'find_root_path' function in /pliant/language/startup.c
The value is taken from PLIANT_ROOT environment variable, or if not available, computed through searching for the 'pliant' directory at various places on the disks.
This should be replaced by computing the pliant root path from argument 0 of the command line plus the current working directory if necessary.
The biggest problem for switching is that this part of the C code is far from beeing clean, and the new algorithm needs to defined the get current working directory on various plateforms.

The two other Pliant directories are computed in 'find_computer_name' in /pliant/language/context/computer.pli module.