Properly fail userspace reboot if it's not supported

Previously, after `adb reboot userspace` is called on a device that
doesn't suppor it, init would've logged an error and quietly exit the
shutdown sequence. This was leaving adb handing forever.

With this approach, init will fail setprop
"sys.powerctl=reboot,userspace" in case userspace reboot is not
supported.

Test: adb root
Test: adb setprop init.userspace_reboot.is_supported 0
Test: adb reboot userspace
Test: atest CtsInitTestCases
Bug: 146639622
Change-Id: I1264078f53ed3ff54638c7f3b6846b7437f98ee5