init: Terminate gracefully when CAP_SYS_BOOT is absent

This change makes it possible for Android running in a container to
terminate cleanly instead of calling abort() when requested to shut
down.

Bug: 62388055
Test: setprop sys.powerctl reboot makes init terminate nicely

Change-Id: I31c7b475d89d7cbd665e135d9b8951dfd4bca80d