devproc: return process id when reading /proc/n/ctl file

allow reading the control file of a process and return
its pid number. if the process has exited, return an error.

this can be usefull as a way to test if a process is
still alive. and also makes it behave similar to
network protocol directories.

another side effect is that processes who erroneously
open the ctl file ORDWR would be allowed todo so as
along as they have write permission and the process is
not a kernel process.

sys/src/9/port/devproc.c

@@ -442,6 +442,7 @@ procopen(Chan *c, int omode0)
 			error(Eperm);
 		break;
 
+	case Qctl:
 	case Qargs:
 	case Qwait:
 	case Qnoteid:
@@ -458,11 +459,6 @@ procopen(Chan *c, int omode0)
 		pid = p->noteid;
 		break;
 
-	case Qctl:
-		if(p->kp || omode != OWRITE)
-			error(Eperm);
-		break;
-
 	case Qmem:
 	case Qregs:
 	case Qfpregs:
@@ -946,6 +942,9 @@ procread(Chan *c, void *va, long n, vlong off)
 		}
 		error(Ebadarg);
 
+	case Qctl:
+		return readnum(offset, va, n, p->pid, NUMSIZE);
+
 	case Qnoteid:
 		return readnum(offset, va, n, p->noteid, NUMSIZE);