Use the IP_FREEBIND setsockopt(2) option where available

[Castaglia]

The IP_FREEBIND socket option, where available, allows for binding to IP address which does not (yet) exist.

This is useful, for example, for daemons integrating with systemd. When proftpd wants to bind to particular IP address during system start, there is no way how to determine that the network interface is really configured and the IP address is available.

See the similar request made for Apache httpd to support this: https://bz.apache.org/bugzilla/show_bug.cgi?id=58725

[Castaglia]

Fixed in master.