readlink `/proc/self/exe` returns absolute path

Problem: `readlink /proc/self/exe` returns relative path if the executable begins with `./` under Parrot

Solution: Under Parrot, when the access path of readlink system call is `/proc/self/exe`, Parrot will call `pfs_process_lookup(pid)` to obtain the `pfs_process` struct and store the file name referred by `/proc/self/exe` into `target->name`. 
What we need to do is ensure the `target->name` to be absolute path.

fixes #348.