Simplify the meaning of return values from the inout handlers. After this
change 0 means success and non-zero means failure.

This also helps to eliminate VMEXIT_POWEROFF and VMEXIT_RESET as return values
from VM-exit handlers.

CR:		D480
Reviewed by:	grehan, jhb
Insert new threads at the end of the thread list in the process
instead of at the beginning.  This allows an intra process signal
to be sent to the oldest thread with the signal unmasked - which,
if it still exists, is the main thread.  This mimics behavior
found in Linux and Solaris.