init: Combine two global sigchld_fd variables into one

Remove the Service::SetSigchldFd() method. Make the Service::GetSigchldFd()
create a signalfd for SIGCHLD. This makes it possible to use a SIGCHLD
signalfd in unit tests.

Change-Id: I0b41caa8f46c79f4d400e49aaba5227fad53c251
Signed-off-by: Bart Van Assche <bvanassche@google.com>