Correct TTY type detection

This reuses the trick used within OTP to pick within old and new shell.

The 'user' structure is the same for all cases (escript, escript + dumb
TERM, unstable install, unstable install + dumb TERM), so we take it
down first.

Then we boot the TTY driver, which fails if TERM=dumb, in which case we
boot the retro-style usr. If it worked, we shut down the driver again,
and boot a modern shell structure.

This avoids all warnings and seems to work in all cases.

src/rebar_prv_shell.erl

@@ -112,36 +112,42 @@ info() ->
     "Start a shell with project and deps preloaded similar to~n'erl -pa ebin -pa deps/*/ebin'.~n".
 
 setup_shell() ->
-    case process_info(whereis(user), current_function) of
-        {_,{user,_,_}} -> setup_old_shell();
-        _ -> setup_new_shell()
-    end.
+    OldUser = kill_old_user(),
+    %% Test for support here
+    NewUser = try erlang:open_port({spawn,'tty_sl -c -e'}, []) of
+        Port when is_port(Port) ->
+            true = port_close(Port),
+            setup_new_shell()
+    catch
+        error:_ ->
+            setup_old_shell()
+    end,
+    rewrite_leaders(OldUser, NewUser).
 
-setup_new_shell() ->
-    %% scan all processes for any with references to the old user and save them to
-    %% update later
+kill_old_user() ->
     OldUser = whereis(user),
-    %% terminate the current user
+    %% terminate the current user's port, in a way that makes it shut down,
+    %% but without taking down the supervision tree so that the escript doesn't
+    %% 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!
+    OldUser.
+
+setup_new_shell() ->
+    %% terminate the current user supervision structure
     ok = 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)
     ok = wait_until_user_started(3000),
-    NewUser = whereis(user),
-    rewrite_leaders(OldUser, NewUser).
+    whereis(user).
 
 setup_old_shell() ->
     %% scan all processes for any with references to the old user and save them to
     %% update later
-    OldUser = whereis(user),
-    %% terminate the current user's port, in a way that makes it shut down,
-    %% but without taking down the supervision tree so that the escript doesn't
-    %% 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!
     NewUser = rebar_user:start(), % hikack IO stuff with fake user
     NewUser = whereis(user),
-    rewrite_leaders(OldUser, NewUser).
+    NewUser.
 
 rewrite_leaders(OldUser, NewUser) ->
     %% set any process that had a reference to the old user's group leader to the