This silences the pedantic warning messages from gcc 4.8.3

add swap instruction for allocator field

use CV_CUDA_TEST_MAIN for opencv_test_core to initialize CUDA
device information

it allows to use cudaHostAlloc methods for cv::Mat objects

* it is the only place, where it is used
* no need to make this function public

move main CUDA group to modules/core/cuda.hpp

add a note to use new cudev module as a replacement

it is internal class, no need to export it

The deinitialization of BufferPool internal objects is controled by global
object, but it depends on other global objects, which leads to errors
caused by undefined deinitialization order of global objects.

I merge global objects initialization into single class, which performs
initialization and deinitialization in correct order.