Fix sigchld-Handler

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.

Backport of the patch that was added to qtbase (Qt5) in
  97279d05822a70da1fb3dab083d823a5f5a008fe
also contains the fix from
  f83fa3c95e9ac6badc393c198c8bc08bc45bea96
.

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