client: Forbid SSH password authentication except for "isr auth"
client: Forbid SSH password authentication except for "isr auth"

- Prevent SSH from asking for a password if the user has not run "isr auth",
since this behavior can be confusing (and annoying, since it will prompt
once per rsync or srun() call).  Instead, fail SSH-based operations (other
than "auth") if the user is not authenticated.

cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr