Add support for installing in non user-writable location when using Federated ID to log in 

Currently, when using Federated ID, `sshproxy.sh` will write files `accesstoken.txt` and `server.log` to the program directory. This prevents installation in locations like `/usr/local/bin` (Linux/MacOS) where a user may not have write access.

Perhaps create a directory $HOME/.sshproxy for those files instead so that it is possible to install the scripts in a non-user-writable location?