nixos/prosody: add ExecReload

Add an ExecReload command to the prosody service, to allow reloading
prosody by sending SIGHUP to the main process, for example to update
certificates without restarting the server. This is exactly how the
`prosodyctl` tool does it.

Note: Currently there is a bug which prevents mod_http from reloading the
certificates properly: https://issues.prosody.im/1216.