this is important to make it easier to guess the ssh port from the
central server.
we rely on ServerAliveInterval instead to reconnect when we lose the
server.
this was unintentionally removed in november 2012 in the isuma-autossh
package, saying it was "not supported everywhere" and due to some
confusion about the defaults (defaults are to *enable* the port). see
commit ec0ebdd9533a29ee4f62f9fbb84ee9e80219ef84 in there.
6ea0beb114