[mc] ptrace the model-checker application

The first goal is to be able to detect crashes of the model-checked
application in the model-checker. This first patch only implement the
ptrace but does not detect the crashed properly yet.

As we are using ptrace, we do not need to using messages to
stop/resume the model-checked application. The HELLO message is
removed whose only purpose was to synchronise the execution of the two
processes (wait for the model-checked application to be ready before
trying to read its memory map).