[mc] Create a separate simgrid-mc program

[mc] Create a separate simgrid-mc program

We create a separate program for the model-checker. The goal is that
this program will:

  - prepare the environment for the child/main process (environment
    variables, maybe LD_PRELOAD a library, pass file descriptors);

  - hold all the model-checker state;

  - communicate with the child process;

  - handle some part of the snapshoting/restoration logic;

  - handle the state comparison logic.

Currently it only enables the custom heap in the child process.