[waitpid] Fix bug ignoring WUNTRACED and WCONTINUED
[waitpid] Fix bug ignoring WUNTRACED and WCONTINUED

Because of a bug, the extra options WUNTRACED and WCONTINUED were
ignored by sys_waitpid() but, tests passed anyway because
wake_up_tasks_waiting_on() did not check for WEXTRA_* flags in
the case parent's waiting for multiple children (-> pid = -1).
This patch makes waitpid() set wobj's extra flags and its wakeup
function (wake_up_tasks_waiting_on()) to check for that.

vvaltchev/experimentOs,vvaltchev/experimentOs,vvaltchev/experimentOs,vvaltchev/experimentOs