This reverts commit 21605d2c.

We're doing this because we wanted the configuration of proxy rules
to be a 2-tuple, as in `[ "22", "127.0.0.1:22" ]`.  But `config`
couldn't handle that before, so we temporarily changed it to
a string, as in "22 => 127.0.0.1:22".

Closes #1058.