fix lua libraries loading (so we don't need to clean the stack)

- split luaopen_ulatency into luaopen_ulatency, luaopen_u_proc,
  luaopen_u_task and luaopen_uflag loaders
- make them return created table on the lua stack
- open them with lua_call()
remove broken libcgroups support
remove lbc library (not used yet)

A simple arbitrary precision library for Lua, based on code from GNU bc
https://github.com/LuaDist/lbc/
definitely remove coreutils lib from source tree
fix CMake variable CONFIG_PREFIX
change the type of CMake variable SYSTEMD_DIR from STRING to PATH
reformat README
README: update example of ulatencyd invocation
CMake: improve libprocps configuration

- add FindProcps.cmake module