After JDK-8376959 the only use of libcrexec is running CRIU; in fact the code already is quite CRIU-dependent. The point of this task is to stop pretending that libcrexec is generic, and move code from criuengine binary (now removed) into libcriuengine implementing the C/R API.

This removes smuggling of some parameters through environment variables and execution of the criuengine. We still require anexecuteable (now called `criuhelper`) to become parent of the restored process, but this has significantly simplified implementation.

Replacement of communication between the restoring and restored process through signals and temporary files is out of scope of this change.