Windows clients and PuTTY

[David Cheney]

> That seems pretty valid, and with a decent error message when we can't find
> it,

exec.LookPath("putty.exe") should do for a first blush. We can
interpret any error from that function as having a high probability
that putty isn't found.

it should be pretty easy for the user to fix (could we give a link the
> website, or is that not kosher?)

Dunno, possibly we send them to a page on the docs website and defer
the problem (sorry Nick)