ROBUSTNESS: Assert that all paths set exist

[HenrikBengtsson]

To prevent mistakes such as #94 from happening, assert that all PATH, LD_LIBRARY_PATH, MANPATH etc. have existing folders.

This can be done:

1. by make check

2. at run-time when the module is loaded

[HenrikBengtsson]

The easiest is to do it during checks. Then one can set up a generic test used by all modules.

[HenrikBengtsson]

Implemented path validation via make check.