reboot: only pause indefinitely for non-shutdown operations

If -p flag specified, return immediately so that scripting code
can progress.  Shutdown is unique in that if it does happen,
the device will never come back, but if it fails one can continue
diagnosis (without the need for a script to send a SIGINT to break
out of the indefinite pause on the reboot -p command).

Yes, this will break adb shell reboot -p ; adb wait-for-device as
noted in the comment, but no one should ever expect the device to
come back if a shutdown is requested.  We do not break adb reboot ;
adb wait-for-device sequence though as we retain the pause forever.

Test: manual
Bug: 63736262
Bug: 38446744
Bug: 66912053
Change-Id: I028cd873a7193a78c6b3c342eca1e08b6b296fd2