Mutithreading testing area.

Since this stuff is very very platorm specific, this is not part of the
normal build.  Have a read of doc/threads.doc.

mttest will do some testing and will currently build under Windows NT/95
and solaris.  The IRIX stuff is not finished.

I have tested this program on a 12 CPU ultra sparc box (solaris 2.5.1)
and things seem to work ok.
