cache logging fixes

[matthiasdiener]

• reflect that LOOPY_NO_CACHE/PYOPENCL_NO_CACHE use strtobool now
• fix actx_class_is_pyopencl
• POCL_CACHE_DIR overrides XDG_CACHE_HOME for pocl

Questions for the review:

• [x] Is the scope and purpose of the PR clear?
  • [x] The PR should have a description.
  • [x] The PR should have a guide if needed (e.g., an ordering).
• [x] Is every top-level method and class documented? Are things that should be documented actually so?
• [x] Is the interface understandable? (I.e. can someone figure out what stuff does?) Is it well-defined?
• [x] Does the implementation do what the docstring claims?
• [x] Is everything that is implemented covered by tests?
• [x] Do you see any immediate risks or performance disadvantages with the design? Example: what do interface normals attach to?