Fix NativeProcessLinux thread stop notification.

NativeProcessLinux now notifies the delegate of the eStateStopped
only after all threads intended to stop have really hit their
stop signal.

NativeProcessLinux tracks each thread id it expects to have stopped
in a synchronized unordered thread id set.  Resume () now collects
all threads meant to be stopped.  It locks the tid set, fires off
the tgkill on the tid, and adds the tid to the set.  The signal
handling for a tgkill stop locks the mutex, removes the tid, warns
if the tid wasn't there, and then notifies the delegate that it stopped
only when the wait-for-stop tid set is empty.

Any place a thread can get removed will need to clean up its entry
from this set.