Listen for UDP packets on all network interfaces

[tslocum]

GGPO-Go specifies the address 0.0.0.0 which only allows local connections. To also allow external connections, no address should be specified.

net.ListenPacket docs:

// See func Dial for a description of the network and address
// parameters.

net.Dial docs:

// For TCP, UDP and IP networks, if the host is empty or a literal
// unspecified IP address, as in ":80", "0.0.0.0:80" or "[::]:80" for
// TCP and UDP, "", "0.0.0.0" or "::" for IP, the local system is
// assumed.