nusb/usbd: cleanup processes on unmount

this makes sure that when postsharesrv() fails (for
example because the shr file already exists), the
worker process gets killed and all file descriptors
to devusb get closed.

sys/src/cmd/nusb/usbd/usbd.c

@@ -338,8 +338,15 @@ usbdstart(Srv*)
 	}
 }
 
+static void
+usbdend(Srv*)
+{
+	postnote(PNGROUP, getpid(), "shutdown");
+}
+
 Srv usbdsrv = {
 	.start = usbdstart,
+	.end = usbdend,
 	.attach = usbdattach,
 	.walk1 = usbdwalk,
 	.read = usbdread,