Implement reforks by forking the application, to save the app exec time

Implement reforks by forking the application, to save the app exec time

Instead of forking from the checker and exec()ing the application, we
now fork+exec an application that we use as a proxy to the real
application process.

When we need a new application process, we fork it from the proxy,
which is much faster as it's already initialized.

The extra complexity is when the socket closes abruptly on the Checker
side, which means that the application died. We cannot waitpid() on
the app directly, as it's our grandchild. So, we have to ask the proxy
to do the waitpid for us and return the status.