init: Prevent spin loop while waiting for exec or property

Currently, when we are waiting for an exec service or a property, if
there are either any services to be restarted or any more commands to
be run, we set the epoll_timeout to handle these events.  However, we
don't actually restart and processes or execute any commands while
waiting, so this essentially turns this waiting into a spin loop,
particularly in the common case of having more commands to execute,
where epoll_timeout is set to 0.

The change only sets epoll_timeout if we're not waiting.

Note that the only way to stop waiting for an exec service or a
property is for a signal or property to be delivered to init, which
happens through the epoll fds, so it's safe to indefinitely wait for
epoll to return.

Test: Boot bullhead
Change-Id: Iae3b217eb28182038b464fd39df8e7d27b5e23ff