added dedicated <modname>_init.cpp files with initModule_<modname>() functions and all the relevant structures; made BackgroundSubtractorMOG/MOG2 derivatives from Algorithm; cleaned up MOG2 implementation and made it multi-threaded.

HAVE_CONFIG_H -> HAVE_CVCONFIG_H

a lot of small corrections to bring down the number of undocumented functions, reported by the script; added em.cpp sample

replaced alloca() (a.k.a. cvStackAlloc) with AutoBuffer or vector() everywhere. cvStackAlloc() is still defined, but we do not need alloca() anymore to compile and run OpenCV (fixes #889 and may be some others)

revert to using fixed RNG in EM, otherwise the algorithm becomes non-deterministic and the test fails sometimes. in the meantime, EM needs to switch to more robust cv::kmeans instead of CvEM::kmeans.