sig_handle_first_stop(): notify debugger

[kib]

Summary

Since it could be that no debuggee threads are waken up because all of
them are in interruptible sleep and consequently all were suspended
remotely, call thread_stopped().

Tested by:      pho

sig_suspend_thread(): return info about the way the process was suspended

so that caller can infer whether the thread_stopped() is to be called by
the AST handler.

[kib]

Only create a notification for debugger if no thread is going to call thread_stopped() because all of them were suspended remotely.