Display proxy ephemeral port when the user specifies 0

[jonatanklosko]

The current behaviour is to print 0, which is not meaningful:

$ fly proxy 0:5558 ...
Proxying local port 0 to remote [...]:5558

With this change we special case port 0 and print the actual ephemeral port assigned to the listener.

[dangra]

thanks @jonatanklosko