* Reads the client key from /gluetun/client.key
* Read the client certificate from /gluetun/client.crt
* Additional checks for client key and client certificate validity
* Fix client key file parsing if environment variable isn't present
- Fix bad permissions bits for files
- VPNSP is 'private internet access' instead of 'pia' (retro compatible)
- Check errors of deferred unsetEnv functions in params package
- Other lint errors fixing and code simplifications