Fix for PATH_ADDON encoding

This checks the system encoding and decodes the current working
directory into a unicode string based on that, fixing start-up crashes
when the add-on tries to read the configuration file.
Use system temporary directory

This has been put in as a fix for Windows users who cannot send
non-ASCII paths via CLI interfaces, which affects things like
transcoding. By using the system temporary directory, we can later, in
Python, move the file to its correct place.