eventlet backdoor doesn't always play nicely with gettext

When using the eventlet backdoor for troubleshooting, it can sometimes interfere with the global _ symbol that gettext installs. It can often manifest itself as an exception similar to this:

TypeError: 'dict' object is not callable

With the type varying.

The problem is the default sys.displayhook function. It will take the return value of the last expression, print it and then set it to __builtin__._, which is also the variable that gettext sets to allow _() to be used to internationalize strings.

An attempt to workaround the problem was implemented, but it does not always work.

The fix is to implement a new sys.displayhook that plays nicer with gettext.