run daemon as the irker user (Close: #832758)

this is an improvement upon the default .service file. it requires a irker user to be created, something which is automatically handled by the debian package, but should be handled by other distributions when deploying the .service file.

there are obvious dangers in running irkerd as root: a compromise would be catastrophic, and since it runs on public servers that are traditionnally pretty hostile (IRC), it seems critical that rights of the daemon be limited.

#832758 is a reference to the Debian bug where this issue was first documented.

merged

mentioned in commit drnlmza/irker@3db4a034