Packages: moved systemd service to forking on rpm-based distros

[thresheek]

Fixes #915
Fixes #1178

[ac000]

Hi @thresheek

Please replace the Fixes tags with

Closes: https://github.com/nginx/unit/issues/915
Closes: https://github.com/nginx/unit/issues/1178

We use Fixes for when referring to an actual commit, e.g

Fixes: c1e3f02f9 ("Compile with -fno-strict-overflow")

And use full URLs so they are actionable from outside GH, e.g from a terminal.

(Note: While we normally put URLs inside <> we don't for these tags as GH doesn't recognise them if we do)

[thresheek]

Thanks, I've fixed the commit message as per your guidelines.