Add Server available interfaces IP address in Verbose mode

[davidBar-On]

• Version of iperf3 (or development branch, such as master or
  3.1-STABLE) to which this pull request applies:
  master

• Issues fixed (if any): Add the IP address when the server starts #1845

• Brief description of code changes (suitable for use as a commit message):

Suggested way for the Server to display the IP addresses is is listening to, after displaying the "Server listening on ..." message. For example:

-----------------------------------------------------------
Server listening on 5201 (test #1)
Available Interfaces: lo__127.0.0.1
                      lo__10.255.255.254
                      eth0__123.45.67.204
                      lo__::1
                      eth0__ab80::123:cdff:efa3:b7cd
-----------------------------------------------------------

Few comments to the approach used:

1. The IPs are displayed only in Verbose mode, to keep backward compatibility. It may be that displaying the data always is better.
2. The IPs display format is: "<interface>__<ip addr>", as I didn't find a "standard" way to display this information. The "__" separator is used and not ":", since ":" can be confused with the "::" in IPv6 addresses.
3. I tested the code in Ubuntu Linux and Windows (Cygwin), but my understanding is that getifaddrs() should be available on all OSs.