Enable libcurl as the network backend

[elecro]

So this patch set will allow WebKitNix to be build with libcurl thus replacing libsoup as the network backend.

By default it will still be built with libsoup but there is a ENABLE_SOUP in the FeaturesNix.config file which if set to 0 will disable the libsoup and thus WebKit will be built with the libcurl.

[hugopl]

I have the dream of a having Nix free of glib in the future, do you think this is reasonable for libCurl backend?

I'm not saying to remove the GIO code now, I'm just asking if this would be possible without too much work in the future.

[balazs]

Definitely, I believe this an important motivation behind moving to libCurl. If I recall correctly it does not have any dependencies beside libc.

[hugopl]

Nice, my advice is: Don't worry with it (glib) right now, one step at a time :-)

[elecro]

At first I wanted to use simple socket calls (for the stream handler and the inspector server), but this was way faster :) . Only one thing is bit tricky and that is the handling the socket data in an async fashion (what the GIO does easly in this case)

[elecro]

So... can we go forward with this?

[hugopl]

Merged on commits: 6c6538e and b793fd3

[kromain]

Yay! Nice job guys, I'll use that as the baseline for my pull request adding WebSockets/InspectorServer support