Distinguish user-requested shutdown from power-related ones

With this patch, when the user requested shutdown,
PowerManagerService sets sys.powerctl is set to
"shutdown,userrequested", and init runs fsck on shutdown.

When shutdown is triggered due to a low power state etc.,
the service sets the property to "shutdown,", and init
immediately shuts down the system without running the
command.

This is a follow-up CL for http://r.android.com/158525.

Bug: 21853106
Change-Id: Iae72990130fe9aa479c802f77301438190dbbfb3