Fix sigchld-Handler

Changed the sigchld-Handler such that the SA_SIGINFO flag is handeled
correctly. Furthermore the signal mask is preserved such that the original
signal handler is not interrupted when not allowed.

Task-number: QTBUG-32979
Change-Id: Iec7663e7289ea5d95155f52cf8788ebf646cfabd
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>