systemd.py: Fallback for openpyn_path when not in PATH

[AbdealiLoKo]

Earlier we were assuming that openpyn_path was always found in the PATH and we got that from shutil.which() But if it is not found, we assume that the command being run right now is the path to the executable.

Handles cases like: sudo /bin/openpyn --init where the user explicitly gives an abspath

Resolves: https://github.com/jotyGill/openpyn-nordvpn/issues/289