Disable I/O to the console if the old-style Erlang console is used

There's a nasty problem with the Erlang VM + lager when the old-style
Erlang console is used.  You can use the "-oldshell" flag to explicitly
get the old-style shell.  However, if the Erlang VM is started when
*not* associated with a pseudo-tty, the VM will silently use the
old-style shell (because the new-style shell requires a pty to
support command line editing, etc.).  The most common way of starting
the Erlang VM without a pty is to start it via a non-interactive
SSH session.

This patch is opinionated in what to do in the case when the old-
style shell is detected.  My opinion is:

1. Shout loudly to the console log (in a separate Erlang process, so
   that the main lager event handler proc won't be blocked by any
   problems with the old-style console).  It's almost certainly
   likely that you really don't want to run Erlang with the old-style
   shell.  But many sysadmins don't look closely at their systems'
   log files, so (for example) shouting the same message 10x in a row
   is legit.  Reviewers: YMMV.
2. Set a SASL alarm.  Again, many sysadmins' are bad at looking at
   log messages.  Setting a SASL alarm is another method to try to
   get their attention.

To test, create a test script called `/tmp/testit.sh`, changing the
`-pz` parameter to point to the correct place for lager's `ebin`
subdirectory:

    #!/bin/sh

    erl -pz /Users/fritchie/b/src/lager/ebin -eval '{application:start(sasl), lager_console_backend:init([error, true]), timer:sleep(5*1000), io:format("\n\nInfo: ~p\n\n", [{{alarms, alarm_handler:get_alarms()}, {user_drv, whereis(user_drv)}}]), erlang:halt()}.'

Then run it twice, using the following:

    First time: ssh -t localhost sh /tmp/testit.sh
    Second time: ssh localhost sh /tmp/testit.sh

The last lines of the first time should look like:

    Info: {{alarms,[]},{user_drv,<0.22.0>}}

The last lines of the second time should look like:

    Info: {{alarms,
               [{lager_console_backend,
                    "WARNING: old-style console is in use, so lager_console_backend log output to the console is disabled.  Restart the VM on a pseudo-tty to ensure use of the new-style VM console."}]},
           {user_drv,undefined}}

src/lager_console_backend.erl

@@ -40,6 +40,13 @@ init([Level, true]) -> % for backwards compatibility
 init([Level,false]) -> % for backwards compatibility
     init([Level,{lager_default_formatter,?TERSE_FORMAT ++ [eol()]}]);
 init([Level,{Formatter,FormatterConfig}]) when is_atom(Formatter) ->
+    IsSafe = case is_new_style_console_available() of
+                 false=Res ->
+                     spawn(fun warn_user/0),
+                     Res;
+                 Res ->
+                     Res
+             end,
     Colors = case application:get_env(lager, colored) of
         {ok, true} -> 
             {ok, LagerColors} = application:get_env(lager, colors),
@@ -47,8 +54,10 @@ init([Level,{Formatter,FormatterConfig}]) when is_atom(Formatter) ->
         _ -> []
     end,
 
-    try lager_util:config_to_mask(Level) of
-        Levels ->
+    try {IsSafe, lager_util:config_to_mask(Level)} of
+        {false, _} ->
+            {error, "Old style console was detected"};
+        {true, Levels} ->
             {ok, #state{level=Levels,
                     formatter=Formatter, 
                     format_config=FormatterConfig,
@@ -108,6 +117,36 @@ eol() ->
             "\r\n"
     end.
 
+-ifdef(TEST).
+is_new_style_console_available() ->
+    true.
+-else.
+is_new_style_console_available() ->
+    %% Criteria:
+    %% 1. If the user has specified '-noshell' on the command line,
+    %%    then we will pretend that the new-style console is available.
+    %%    If there is no shell at all, then we don't have to worry
+    %%    about log events being blocked by the old-style shell.
+    %% 2. If the user_drv process iss registered, all is OK.
+    %%    'user_drv' is a registered proc name used by the "new"
+    %%    console driver.
+    init:get_argument(noshell) /= error orelse
+        is_pid(whereis(user_drv)).
+-endif.
+
+warn_user() ->
+    Msg = lists:flatten(
+            io_lib:format("WARNING: old-style console is in use, so ~s "
+                          "log output to the console is disabled.  "
+                          "Restart the VM on a pseudo-tty to ensure "
+                          "use of the new-style VM console.",
+                          [?MODULE])),
+    catch alarm_handler:set_alarm({?MODULE, Msg}),
+    [begin
+         error_logger:warning_msg(Msg),
+         timer:sleep(1000)
+     end || _ <- lists:seq(1, 10)].
+
 -ifdef(TEST).
 console_log_test_() ->
     %% tiny recursive fun that pretends to be a group leader