zshrc: rejigger ssh agent stuff

The way I had it before, if I didn't have *all* of my keys (personal and
ndn) active I'd get hounded to unlock all my keys (even ones that were
still active) every time I initialized a new shell. This way it only
tries to activate my personal keys by default but will activate ndn when
I try to ssh to fubar or yakko.

Also retabbed.