Add support for configuring permit-foreign-addresses.

We're building a highly available (-ish) and scalable ftp server cluster, in which we route the control connection through HAProxy. In this mode, the remote address for the control connection is the load balancer, but we're still using direct connections for the data connection. However, by default `pyftpdlib` checks the remote address for all data connections (in `PASV` mode). For this setup, `pyftpdlib` needs to be instructed to accept 'foreign connections'.

This patch adds the ability to configure `permit-foreign-addresses`, so it can be used in this specific setup. This opens up the ability to perform data-connection hijacking, so it's not a good idea to enable this by default. However, the traditional loadbalanced setup, opens up the same risk.