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 helps make BOSH and WebSocket connection endpoints
discoverable via the HTTP method described in
[XEP-0156](https://xmpp.org/extensions/xep-0156.html#http).