Don't go busy loop when waiting child process.

Before this change, code works like this: after gets POLLHUP from poll, it just
go a busy loop to do poll -> waitpid -> poll -> waitpid ... until child process
really exit. This doesn't make sense and is waste of CPU.

Change-Id: If6cd8b0245587d623e309c3ae27f162e1c7ef504
Signed-off-by: Tao Wu <lepton@google.com>