pv-runtime should distinguish between different variants of library search paths

The following discussion from !338 (merged) should be addressed:

Ideally I'd prefer to have pv_runtime_set_search_paths() build three versions of the search path - one for the initial LD_LIBRARY_PATH to run the adverb, one for the final LD_LIBRARY_PATH after the ld.so.cache has been regenerated, and one for --add-ld.so-path - so that all the logic for deciding which paths go where can be in the runtime.