mod_storage_appendmap: Fix keyvalue writes store_raw() takes a filename, unlike most other datamanager functions that take a series of path components, leading to this bug where it would write the hostname into a file named after the first argument (commonly a username) in the working directory.

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

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