function cbListen () {
    if (typeof (port) === 'number') {
      self.host = self.address().address
      self.port = self.address().port
    } else {
      self.host = port
      self.port = self.server.fd
    }

    if (typeof (callback) === 'function') { callback() }
  }
Adds a handler (chain) for the LDAP unbind method.

This method is different than the others and takes no mount point, as unbind
is a connection-wide operation, not constrianed to part of the DIT.

@return {Server} this so you can chain calls.
@throws {TypeError} on bad input