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

service/debugger/debugger.go

@@ -200,9 +200,6 @@ func (d *Debugger) Restart(pos string) ([]api.DiscardedBreakpoint, error) {
 	}
 
 	if !d.target.Exited() {
-		if d.target.Running() {
-			d.target.Halt()
-		}
 		// Ensure the process is in a PTRACE_STOP.
 		if err := stopProcess(d.ProcessPid()); err != nil {
 			return nil, err