errno can be set to a non-zero value from a previously failed system
call, it is not reset by successful ones
=> have to do that manually or the unit test fails because it detects
an "outdated" error message
This change define explicitly the behavior that exiv2::getEnv should have
on response to unexpected inputs.
There are some other minor changes:
- Use _putenv_s for the unit tests on Windows
- Add todo comment
- Remove deprecated note about freeing memory
Luis Díaz Más
Dan Čermák