Fix.listen ipv6

[newgenid]

Hello.

fmt.Sprintf("%s:%d", addr, port) works incorrect for ipv6 addr.
For example, if addr = "::", port = 444, this code returns ":::444". The ":::444" is incorrect address for using in reuseport.ListenPacket().

net.JoinHostPort() supports ipv6 addresses and returns "[host]:port".

Without this fix UDPReceiver does not work with ipv6 addresses.

Thank you.

[lspgn]

Thank you for the details
I could have reopened #316 but will move forward with this one once I check it

[lspgn]

Ok I dived into the code. Would be great to also add ways to break the application.
Ended up finding a different bug and was fearing a regression.

• it won't break passing a -listen sflow://locahost:6343
• but there is no need to use the net function. It just checks if : are present
• currently, the following -listen sflow://::1:6343 is not detected as a bad arguments to GoFlow2
  • the errors are not being intercepted properly

[lspgn]

Should be fixed with #323

[newgenid]

Thank you.