mod_client_certs: do not crash on plain sockets In some situations (e.g., reverse-proxied websocket), non-TLS sockets can be marked as secure, causing mod_client_certs to call the undefined method getpeercertificate and crash.

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

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