init: add reboot_on_failure service option

This replaces the recently added `exec_reboot_on_failure` builtin, since
it'll be cleaner to extend service definitions than extending `exec`.
This is in line with what we decided when adding `exec_start` instead
of extending `exec` to add parameters for priority.

Test: `exec_start` a service with a reboot_on_failure option and watch
      the system reboot appropriately when the service is not found and when
      the service terminates with a non-zero exit code.

Change-Id: I332bf9839fa94840d159a810c4a6ba2522189d0b