init: Reland "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: `adb reboot` on bullhead causes no kernel panics
Test: `adb reboot` on a system without CAP_SYS_BOOT makes init terminate
       nicely

Change-Id: I36b2298610f5b4a2bf8b05103d04804883df2c88