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

[ghweber]

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?