libthread_xu - Clean up the red zone on library unload

libthread_xu - Clean up the red zone on library unload

* If the main program is not linked against -pthread but dynamically
  loads a shared library that is via dlopen(), then dlclose()'s it,
  libthread_xu leaves red-zone and cached thread stacks dangling.

  The second attempt to (indirectly) load libthread_xu then fails with a
  red-zone panic.

* Add a destructor for libthread_xu to unmap the red-zone and to clean out
  any cached thread stacks.

* Note that neither libthread_xu nor most other large libraries are designed
  to be unloaded.  That is, even if loading/unloading works, numerous libraries
  will almost certainly leak memory.  This change prevents the fatal error but
  doesn't fix the general memory leakage problem.

  IMHO Only libraries designed to be unloadable should be unloaded.

Reported-by: Vasily Postnicov <shamaz.mazum@gmail.com>