mod_s2s_keepalive: Fix response handler (thanks Ge0rG) So there are no 'iq-{result,error}/host' events, mod_iq only fires events for result stanzas that include the full 'id' attr. This fixes it by hooking the stanza router event.

This module lets you block connections to any remote servers not on a
whitelist.

``` {.lua}
s2s_whitelist = {
    "example.org",
}
```