Set signal handlers, required to be an "init" process in a PID namespace

The first process created in a new PID namespace (pid_namespaces(7)) is the "init" process, for that namespace. Only signals for which the "init" process has established a signal handler can be sent to it. To run virtiofsd with '--sandbox namespace', we need to set handlers to end the child process if the parent dies. Also, in case the PID namespace is set by an external tool. Fixes #23 (closed)