chrootenv: code review

* Wrap LEN macro in parantheses
* Drop env_filter in favor of stateful environ_blacklist_filter,
  use execvp instead of execvpe, don't explicitly use environ
* Add argument error logging wherever it makes sense
* Drop strjoin in favor of asprintf
* char* -> const char* where appropriate
* Handle stat errors
* Print user messages with fputs, not errorf
* Abstract away is_str_in (previously bind_blacklisted)
* Cleanup temporary directory on error
* Some minor syntactic and naming changes

Thanks to Jörg Thalheim and Tuomas Tynkkynen for the code review!