Track whether a process is a zombie or not yet fully built via flags
PS_{ZOMBIE,EMBRYO} on the process instead of peeking into the process's
thread data.  This eliminates the need for the thread-level SDEAD state.

Change kvm_getprocs() (both the sysctl() and kvm backends) to report the
"most active" scheduler state for the process's threads.

tweaks kettenis@
feedback and ok matthew@