start: Detect early failure of the new child

start: Detect early failure of the new child

If the process in the new namespace dies very early
we have currently no chance to detect this.
The parent process will just die due to SIGPIPE
if it write to the fd used for synchronisation and
nobody will notice the real cause of the problem.

Install a SIGCHLD handler to detect the death.
Later when the child does execve() to the init within
the new namespace the handler will be disabled automatically.

Signed-off-by: Richard Weinberger <richard@nod.at>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>