cmake: Split -E command implementation into separate source file

Move the cmake::ExecuteCMakeCommand static method and all the static
methods it calls out of the 'cmake' class to a separate 'cmcmd' class.
Build the latter as part of the main cmake executable with cmakemain.cxx
and not in CMakeLib.  Drop unused header includes from "cmake.cxx".

By moving this implementation out of cmake.cxx we avoid carrying it
around in all the executables that use class 'cmake'.  It is needed only
for the main "cmake -E" functionality.