env-util: fix parameter handling of parse_env_extension_hierarchies() + getenv_path_list()

Our coding style dictates we should not clobber return parameters on
failure, and always initialize them on success. Do so here.

This changes getenv_path_list() to return ENXIO if the env var is not
set, which is similar to how we handle this in getenv_bool().

This drops debug logging from parse_env_extension_hierarchies(), since
it's done anyway in getenv_path_list()

Follow-up for: #18018