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/proc_unix_test.go

@@ -25,24 +25,21 @@ func TestIssue419(t *testing.T) {
 		_, err := setFunctionBreakpoint(p, "main.main")
 		assertNoError(err, t, "SetBreakpoint()")
 		assertNoError(proc.Continue(p), t, "Continue()")
+		resumeChan := make(chan struct{})
 		go func() {
-			for {
-				time.Sleep(500 * time.Millisecond)
-				if p.Running() {
-					time.Sleep(2 * time.Second)
-					if p.Pid() <= 0 {
-						// if we don't stop the inferior the test will never finish
-						p.RequestManualStop()
-						p.Kill()
-						t.Fatalf("Pid is zero or negative: %d", p.Pid())
-						return
-					}
-					err := syscall.Kill(p.Pid(), syscall.SIGINT)
-					assertNoError(err, t, "syscall.Kill")
-					return
-				}
+			time.Sleep(500 * time.Millisecond)
+			<-resumeChan
+			if p.Pid() <= 0 {
+				// if we don't stop the inferior the test will never finish
+				p.RequestManualStop()
+				p.Kill()
+				t.Fatalf("Pid is zero or negative: %d", p.Pid())
+				return
 			}
+			err := syscall.Kill(p.Pid(), syscall.SIGINT)
+			assertNoError(err, t, "syscall.Kill")
 		}()
+		p.ResumeNotify(resumeChan)
 		err = proc.Continue(p)
 		if _, exited := err.(proc.ProcessExitedError); !exited {
 			t.Fatalf("Unexpected error after Continue(): %v\n", err)