posix_spawn: If an error is detected in the child process, reap the zombie.

posix_spawn: If an error is detected in the child process, reap the zombie.

Formerly, in this case an error was returned but the pid was also returned
to the application, requiring the application to use unspecified behaviour
(the returned pid in error situations) to avoid zombies.

Now, reap the zombie and do not return the pid.

MFC after:	2 weeks