Saving system SSH_AUTH_SOCK as PREVIOUS_SSH_AUTH_SOCK while using
temporary agent, so we can revert to the system agent afterwards.

Also reverting to underscore separation of MassiveLauncherKey
prefix from the rest of the temporary key file name, just because
I didn't like the look of the new temporary key file names in the
debug log.
Ensuring that sshKeyDist's shutdownReal doesn't get called twice.