Fix bugs/race conditions

Following suggestions from @psyeugenic, this code terminates and waits
for the termination of the port handling IO before booting our own,
which should get rid of annoying warnings.

We also allow for the failure to shutdown the user worker under
kernel_sup, since it is likely not there anymore in many scenarios,
preventing crashes.

src/rebar_prv_shell.erl

@@ -133,11 +133,25 @@ kill_old_user() ->
     %% fully die
     [P] = [P || P <- element(2,process_info(whereis(user), links)), is_port(P)],
     user ! {'EXIT', P, normal}, % pretend the port died, then the port can die!
+    exit(P, kill),
+    wait_for_port_death(1000, P),
     OldUser.
 
+wait_for_port_death(N, _) when N < 0 ->
+    %% This risks displaying a warning!
+    whatever;
+wait_for_port_death(N, P) ->
+    case erlang:port_info(P) of
+        undefined ->
+            ok;
+        _ ->
+            timer:sleep(10),
+            wait_for_port_death(N-10, P)
+    end.
+
 setup_new_shell() ->
-    %% terminate the current user supervision structure
-    ok = supervisor:terminate_child(kernel_sup, user),
+    %% terminate the current user supervision structure, if any
+    _ = supervisor:terminate_child(kernel_sup, user),
     %% start a new shell (this also starts a new user under the correct group)
     _ = user_drv:start(),
     %% wait until user_drv and user have been registered (max 3 seconds)