#1 client should print to stderr when prompting for secret

[Ben Poliakoff]

When prompting for the secret, the shimmer client should probably be
using STDERR, at least if we want to be able to use syntax like this:

ssh user@remote.host -p `./shimmer --open ssh:10000:10999`

The attached one-line patch makes this change.

[Ben Poliakoff]