fix: Allow commenting out backend.cfg

[userwiths]

Regarding issue #950

Did not see any processing regarding if the option is commented out or not. Now we check if the first symbol is a semicolon (;) and in case it is, we return default rather than the commented out option.

I did test this locally, and it seems the issue was purely visual. When the semicolon (;) is present, we can see a message like this [WARN] [second.loc] Backend rewrite config is invalid: Invalid backend string ';conf:phpfpm:tcp:php81:9000'. It must start with 'conf:' In the Command & Control tab which stops the app from using the commented out backend and uses the default one.

[Ernestopheles]

Commenting Out the Option ist useful, as it disables it. Please keep this possibility !