Updated the Ultra Minit HTTP bof exploit

After exploiting this application manually I decided to make this
an MSF exploit, only to find that other people had beaten me to it.
However, the existing exploit was broken in a few ways, and this
commit makes those problems go away. They include:

* Correct use of alpha chars in the buffer leading up to the payload
  which results in bad chars being avoided. Bad chars muck with the
  offsets because they get expanded.
* Adjustment of the payload so that it runs in another thread instead
  of in the thread of the request handler. This prevents the session
  from being killed after the hard-coded 60-second timeout that is
  baked into the application.
* The handler thread terminates itself so that the process doesn't
  crash.
* Extra targets were added based on the machines I had access to.
Apply @jlee-r7's feedback
Land #3027, @OJ's fix for ultraminihttp_bof