curl: Add GnuTLS as an alternative to openSSL

[ikervagyok]

Motivation for this change

Added this option, as the game "Dont Starve" running in fhsUserEnv needs libcurl-gnutls.so.4, which we didn't have.
Also i find that having more choice regarding crypto(-libraries) is better.

It can be pulled even though it's marked WIP, but maybe we could/should add other libraries as well (libreSSL/...)

Things done

• Tested using sandboxing
  (nix.useChroot on NixOS,
  or option build-use-chroot in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • OS X
  • Linux
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

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[mention-bot]

@ikervagyok, thanks for your PR! By analyzing the annotation information on this pull request, we identified @vcunat, @edolstra and @wkennington to be potential reviewers

[edolstra]

I'm fine with including an option for building with gnutls, but not including variants in all-packages.nix. That means more work for our build farm, and confusion for users (since nix-env -qa will show a bunch of apparently identical curl packages).

[globin]

@ikervagyok regarding your comment on building with libressl, it only needs overriding openssl with it so no other changes are necessary.

Otherwise this looks fine since you've addressed @edolstra's comment, is it good to go from your side?

[ikervagyok]

@globin I have nothing, which explicitly needs curlLibreSSL, or any other fork to test such a case. So from my perspective it's good to go. Maybe later other people (or I) add more SSL/TLS implementations.

[grahamc]

@ikervagyok is there work to be done on this PR, or is it complete?

[vcunat]

Merge-squash-ed and staged, with a minor nitpick changed.

[joachifm]

Looks like this was integrated.

[vcunat]

Oh, I forgot this. Maybe I even squashed the merge unintentionally.