proc/*: remove Process.Running

Implementing proc.Process.Running in a thread safe way is complicated
and nothing actually uses it besides tests, so we are better off
rewriting the tests without Running and removing it.

In particular:

* The call to d.target.Running() in service/debugger/debugger.go
  (Restart) can never return true because that line executes while
  holding processMutex and all continue operations are also executed
  while holding processMutex.
* The call to dbp.Running() pkg/proc/native/proc.go (Detach) can never
  return true, because it's only called from
  debugger.(*Debugger).detach() which is also always called while
  holding processMutex.

Since some tests are hard to write correctly without Process.Running a
simpler interface, Process.NotifyResumed, is introduced.

Fixes #830

pkg/proc/interface.go

@@ -52,8 +52,10 @@ type Checkpoint struct {
 // Info is an interface that provides general information on the target.
 type Info interface {
 	Pid() int
+	// ResumeNotify specifies a channel that will be closed the next time
+	// ContinueOnce finishes resuming the target.
+	ResumeNotify(chan<- struct{})
 	Exited() bool
-	Running() bool
 	BinInfo() *BinaryInfo
 
 	ThreadInfo