Bug#278394: PAM not run in single address space

[Sam Hartman <hartmans@debian.org>]

I think this use of threads is relatively safe.

Basically as I understand it, threads are used to allow the event loop
to run while holding for pam conversation functions.


Likely ways such a design could break:

* Allowing the pam authentication thread to escape and somehow  getting into the rest of the code
* Allowing two pam threads to exist
* failing to cleanup the pam thread
* Having some interaction where a PAM module or one of its
  dependencies detects that it is running in a threaded application
  and changes its behavior.

I think all of these are unlikely.  I'd be happy to audit the code to
give a better risk description.  I'll assume such an audit would be
welcome and start working on it.


In many ways I think using threads simplifies the code.  Note for
example that #252676 could not have happened with threads enabled.


--Sam