psgi: Move the destruction of the perl interpreter to the atexit hook + more bugfixes

My previous commit ensured that we called the DESTROY hook but we
wouldn't properly call the atexit hook. Now when you start uWSGI as
instructed in that commit you'll get this:

 * localhost:1234?0:

   $$: Calling DESTROY

 * localhost:1234?1:

   *** psgix.harakiri.commit requested ***
   ...The work of process 523 is done. Seeya!
   523: Calling the atexit hook
   523: Calling DESTROY

 * Ctrl+C (or stop):

   ^CSIGINT/SIGQUIT received...killing workers...
   696: Calling the atexit hook

Before this change we wouldn't call the atexit hook when
psgix.harakiri.commit was requested by calling localhost:1234?1.

To make this work I removed the "if busy do not run atexit hooks"
condition added in 1.4-rc2-246-g499202e. Of course uWSGI is going to
think the worker is "busy", it's busy being destroyed, that doesn't mean
we should skip running the atexit hooks.

We'll still skip them under the "if hijacked do not run atexit hooks"
condition added in that commit, and the "managing atexit in async mode
is a real pain" condition added in 1.0.1-289-gd7e8523. Maybe those are
further bugs that need to be solved, but I don't know how to test those
modes.

Now we'll also call PERL_SET_CONTEXT() and PERL_SYS_TERM() appropriately
during destruction. Note that the latter should only be called once even
if you have multiple interpreters.