init: Always reap processes before handling properties

There is a race that manifests like this:

1) A service dies (not processed by init yet).
2) service_manager processes death notification.
3) service_manager gets checkService and calls init to start service.
4) init gets the ctl.start / ctl.interface_start for the service
   but the service already appears started, so it does nothing.
5) init gets sigchld, but doesn't do anything else to restart the
   service

We can avoid all of this if we already reap pending processes before
handling properties in the main loop of init.  Since reaping the
services calls waitid(), there's no race even if the signalfd for
sigchld hasn't triggered yet.  It also won't cost us much efficiency,
since it's only a single system call.

Test: CF boots, init unit tests pass
Change-Id: Ie24ef406055b283797b41b1821c8ebcccead4db4