compositor: don't pass the signal flags to autolaunched process

Child process may be anything and may rely on the signals that weston has blocked for itself. In our use case it was SIGCHLD that wasn't received by the autolaunched application that caused us a lot of trouble.

It seems that weston actually cleans up the session and the signal flags for weston_client_launch. So I've decided to generalise it into a function and reuse in both cases when we fork a child process.