Experimental use of constructor functions to call init

[sagebind]

As mentioned in #333, this is a potentially helpful addition that ensures that curl is initialized on the main thread by using constructor functions that get called by the OS before the current program's main is called.

This has the advantage that, assuming you are on one of the supported platforms, init() will be called safely, correctly, and automatically without concerning the user about the gotchas.

This does have some disadvantages:

• Constructor functions are always invoked, which means that simply including curl can slow down the startup of a user's program even if curl is only conditionally used, or used later in program execution.
• On platforms without a constructor implementation, the user still needs to initialize curl on the main thread. All the common platforms are covered though, so maybe this is a niche scenario.

There's no additional runtime cost to this implementation except on platforms without a constructor, where we do an extra atomic swap that isn't necessary. This is probably fixable with additional conditional compilation.

[sagebind]

I removed the second atomic so it should be just as efficient as it was before now. I also went ahead and updated the documentation.

[alexcrichton]

👍