poweroff: fix bug in orderly_poweroff()

orderly_poweroff is trying to poweroff platform in two steps:

step 1: Call user space application to poweroff
step 2: If user space poweroff fail, then do a force power off if force param
        is set.

The bug here is, step 1 is always successful with param UMH_NO_WAIT, which obey
the design goal of orderly_poweroff.

We have two choices here:
UMH_WAIT_EXEC which means wait for the exec, but not the process;
UMH_WAIT_PROC which means wait for the process to complete.
we need to trade off the two choices:

If using UMH_WAIT_EXEC, there is potential issue comments by Serge E.
Hallyn: The exec will have started, but may for whatever (very unlikely)
reason fail.

If using UMH_WAIT_PROC, there is potential issue comments by Eric W.
Biederman: If the caller is not running in a kernel thread then we can
easily get into a case where the user space caller will block waiting for
us when we are waiting for the user space caller.

Thanks for their excellent ideas, based on ...