The file transfer proxy will listen on, and report, a set of network interfaces. Sometimes, the proxy
is not reachable on these addresses directly (for instance in a network environment where NAT comes
into play).

This commit re-purposes the 'xmpp.proxy.externalip' property. Earlier, this was used to make the proxy
bind to a specific interface. As this is quite similar to the functionality provided by the
'network.interface' property, 'xmpp.proxy.externalip' now controls only what interface is reported. It
no longer affect the interface binding process directly. This functionality has also been added to the
admin console.