OS: fix waitpid on some Linux
OS: fix waitpid on some Linux

Like on Apple systems, ECHILD should lead to that specific status code.

Change-Id: Icd809e0003eca223ed0327d2be597b0a7184a86b
Reviewed-on: http://gerrit.aldebaran.lan/78624
Reviewed-by: vpaleologue <cb19838a79eb484e12a6960acfbf7c16dfdb80d9@aldebaran-robotics.com>
Reviewed-by: jlamotte <70353e7b0c7d18b7fc75605daf0ccba69fcd3222@aldebaran.com>
Tested-by: vpaleologue <cb19838a79eb484e12a6960acfbf7c16dfdb80d9@aldebaran-robotics.com>

aldebaran/libqi,aldebaran/libqi,aldebaran/libqi