refactor _image_captcha_check_setup() for GD/JPEG/PNG support

Before #579080: image CAPTCHA in PNG format: check GDlib's PNG support, we only used (and checked for) GD-lib's JPEG support, but now, we have both JPEG and PNG support, and at least require one of them. To cover this, a more advanced setup/support checking is required.

_image_captcha_check_setup() in image_captcha.module now only returns the first problem it finds, but this probably need to be refactored/reworked to return a bitfield of detected problems/support.

On the other hand, I think it is pretty safe to assume that JPEG and PNG support will occur mostly together, so only checking for JPEG support also covers PNG support. For this reason, this issue is marked with minor priority for now. However, if this proves to be a real problem "in the wild" (aka bug reports get in), we should give this a more thorough look.