REST API: in the Server header, identify the software, not the machine

[Al2Klimov]

Rationale: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Server

Test

> GET /v1/checker?command=cpu HTTP/1.1
> Host: 10.27.0.147:5668
> User-Agent: curl/8.6.0
> Accept: */*
>
< HTTP/1.1 200 Ok
< Server: IcingaForWindows/1.12.3
< Content-Type: application/json
< Content-Length: 427
<