init: Allows shutting down cleanly.

When ro.build.shutdown_timeout is set, init will send a SIGTERM signal to
all services on reboot. The normal shutdown process will continue once
all services have exited or after the shutdown timeout
(ro.build.shutdown_timeout).

If ro.build.shutdown_timeout is not set, we assume a 0s timeout.

Bug: 26216447
Test: manual: Ask to reboot. All services exit cleanly.

Change-Id: If921f6e8d87211e500ac9fa86f3e1eabe02d18cf