trurl: add --curl to only count as valid URLs supported by libcurl

[emanuele6]

I would prefer that trurl rejected URLs with schemes not supported by libcurl when I use it in combination with curl.

[emanuele6]

Hmm, on CI the error message is This libcurl build doesn't support the given URL scheme [foo://bar], but it is Unsupported URL scheme [foo://bar] with libcurl 8.4.0. :/
I have to use "stderr": true.

[bagder]

Maybe we should do something more generic for the error message checks...

[emanuele6]

Resolved conflicts, and rebased to get the fix from #240

[bagder]

Thanks!