new feature: support importing xban2 dbs

[fluxionary]

this should be fairly straightforward. xban2's "database" is an array of entries like

{
    names = {"foo", "bar", "123.123.123.123"},  -- names & ips mixed
    banned = nil or true,
    reason = nil or "ban/unban reason",
    source = nil or "foo" -- player who banned, or "xban:importer_*",
    time = nil or 12345, -- seconds since epoch as an int
    expires = nil or 12345, -- expiration time in seconds since epoch
    record = {
        { source = "source", reason="ban/unban reason", time = 1234, expires = nil or 1234 }
    }
}

see https://github.com/minetest-mods/xban2