Binding-table generation always skip psid 0

[dpino]

Command generate-configuration always skips psid 0. That's mostly ok for all cases, since psid 0 represents system ports (0-1024). But in the case psid-length is 0 and shift is 16 (that means configurations that give a full IPv4 port-range), psid 0 softwires should not be skipped. Example:

$ sudo ./snabb lwaftr generate-configuration 193.5.1.100 10 fc00::100 fc00:1:2:3:4:5:0:7e 0
softwire-config {
binding-table {
}