athena.py: Refactor preloading of libraries

Move the LD_PRELOAD code into a separate shell script
(athena_preload.sh). This will allow the use of the same preloading
logic in other applications, e.g. the HLT.

While quit a bit of cleanup was done, the code is kept entirely
backwards compatible where it affects the shell environment of child
processes.


Former-commit-id: 5e3a8b2f