Add a hack to deinstall pthread_atexit handlers at dso unload time

[Martin Husemann <martin%duskware.de@localhost>]

I would like to adopt a hack like FreeBSD did here:

https://lists.freebsd.org/pipermail/svn-src-head/2010-August/019849.html

This allows dynamically loaded shared libraries to install
pthread_atexit handlers. Currently there is no way for the lib to
deinstall the handler in time, and later fork()s would crash the app.

The same behaviour (auto deinstall the handlers) has been implemented
in Linux and Solaris as well.

I don't like the solution, but now solving it differently sounds painfull
as well.

What do others think?

Martin